How do some apps overcome phone recording restrictions?

Background

Phone recording is not really supported on Android, yet some devices support it to some extend.

This made various call recording apps gather as much possible information about devices and what should be done to them, and decide upon this what to do.

Some even offer root solutions.

One such example is boldbeast Call Recorder app, which offers a lot of various configurations to change:

"record mode" . Shows 14 modes for non-rooted devices, and up to 34 for rooted. Also shows "Alsa mode" as an option for it, for rooted devices.
Has "Tune Audio Effect ("auto tune a groupd of parameters") .
Has "Tune Audio Route", with the possible values of "Disabled", "Group1", "Group2", "Group3"
For rooted devices:
"change audio controls" ("auto change audio controls")
"change audio driver" (change audio drive settings to enable record mode 21,22,23,24,31,32,33,34")
For rooted devices: "start input stream"

The problem

If I'm in need to create a call recording app, there is no other way than to find the various workarounds for various devices, but as it seems other apps use terms that don't appear in the API.

I can't find any of those of the app I've mentioned, for example.

What I've found

Other than tons of questions of how to record calls on Android, showing that it doesn't work on all devices, I could find some interesting things. Here are my tries and insights so far:

There are some Audio recording sources we can use while preparing the recording (docs here) , but sadly in each device it might be different. For some, VOICE_CALL works, and for some, others. But at least we can try...
On OnePlus 2 with Android 6.0.1, incoming calls can be recorded using VOICE_CALL, but I can't make outgoing calls be recorded there, unless I use MIC as audio source together with speaker turned on. Somehow, the app I've mentioned succeeds recording it without any issues. I'm sure I will see other issues with other Android devices, as I've tried to address this whole topic in the past. Update: I've found this sample project (also here), which for some reason sleeps for 2 seconds on the UI thread between prepare and start calls of the mediaRecorder. It works fine, and when I did something similar (wait using Handler.postDelayed for 1 second), it worked fine too. The comment that was written there is "Sometimes prepare takes some time to complete".
On Galaxy S7 with Android 8, I've failed to get sound of the other side for outgoing calls AND incoming calls (even with MIC and speaker), no matter what I did, yet the app I've mentioned worked fine.
To let you try my POC of call recording, I've published an open source github repository here, having a sample that will record a single call, and let you listen to the most recent one, if all works well.
This "ViktorDegtyarev - CallRecLib" SDK , which doesn't seem to work at all, and crashes on various Android versions
These 2 old sample projects : rvoix , esnyder-callrecorder , both fail to actually record. The second doesn't even seem to work on Android 6.0.1 device, which it's supposed to support.
aykuttasil - CallRecorder sample and axet - android-call-recorder sample - both, just like on my POC, don't have any tweaking except for AudioSource, and because of this they fails to record on some cases, such as OnePlus 2 output-audio of outgoing calls.
Most third party apps only offer the AudioSource tweaking, but some (like "boldbeast") do offer more. One example is "Automatic Call Recorder" which has "configuration" (10 values to choose from, first is "default") and "method" (5 vales to choose from, first is "default"). Those apps probably do not want others to understand what those configurations mean, so they put general names. Or, it's just too complicated for everyone (especially for users), so they generalize the names.
There is an API of "setMode" here, but it doesn't seem to change upon calling it. I was thinking of maybe change the "channel" of where the call is being used, this way, but it doesn't work. It stays on the value of "2" during call, which is MODE_IN_CALL.
There are customized parameters that are available for various devices (each OEM and its own parameters), which can be set here and maybe even via JNI (here and here) , but I don't get where to get this information from (meaning which pairs of key-value are available). I've searched in a lot of places, but couldn't find any website that talks about which possible parameters are available, and for which devices.
I was thinking of using AudioRecord instead of MediaRecorder class for recording, thinking that it's a bit low level, so it could give me more power and access to customized capabilities, but it seems to be very similar to MediaRecorder, and even use the same audio sources (example here).
Another try I had with low level API, was even further, of using JNI (OpenSL ES for Android). For this, I couldn't find much information (except here and here), and only found the 2 samples of Google here (called "audio echo" and "native audio"), which are not about recording sound, or at least I don't see them occur.
Android P might have official way to record calls (read here and here). Testing on my Android P DP3 device (Pixel 2), I could record both sides fine in both incoming and outgoing calls, using "DEFAULT" as audio source, so maybe the API will finally be official and work on all Android versions. I wrote about it here and here.
I was thinking that maybe the Visualizer class could be a workaround of recording, but according to some StackOverflow post (here), the quality it extremely low, so I decided that maybe I shouldn't try it. Plus I couldn't find a sample of how to record from it.
I've found some parameters that might be available on some devices, here (found from here), all start with "AUDIO_PARAMETER_", but testing on Galaxy S7, all returned empty string. I've also found this website, that gave me the idea of using audioManager.setParameters("noise_suppression=off") together with MIC audio source, but this didn't seem to do anything in the case of Galaxy S7.

The questions

As opposed to other similar questions about this topic, I'm not asking how to record calls. I already know it's a very problematic and complex problem. I already know I will have to address various configurations, and that I will probably use a server to store all of them and find there the best match for each one.

What I want to ask is more about the tweaking and workarounds :

Is there a list of configurations for the various devices, Android versions, and what to choose for each?
Besides Audio source, which other configuration is possible to be used?
Which parameters are possible for the various devices and Android versions ? Are there any websites of the OEMs describing them?
What are the various terms in the app I've mentioned? Where can I find information of how to change them?
Which tools are available for rooted devices?
Is it possible to know which device supports call recording and which not, by using the API ?
About the workaround of OnePlus 2, to wait a moment till we start recording, why is it needed? Is it needed on all Android versions? Is it a known issue? Would 1 second be enough?
How come on the Galaxy S7 I've failed to record the other side even when using MIC&speaker?

EDIT: I've found this of accessibility service being able to help with call recording:

https://developer.android.com/guide/topics/media/sharing-audio-input#voice_call_ordinary_app

Not sure how to use it though. It seems "ACR Phone Dialer" uses it. If anyone knows how it can be done, please let me know.

Solution

According to my tests, one way to improve this is to have an AccessibilityService being active (no need to write there anything at all) while choosing voice-recognition as the audio source. Also it's recommended to have the speaker turned on because this will record the audio from the microphone.

This seems to exist in some call-recording apps.

Weird thing is that Google has written this as a rule on the Play Store:

The Accessibility API is not designed and cannot be requested for remote call audio recording.

https://support.google.com/googleplay/android-developer/answer/11899428

No idea what the "remote" means here.

Anyway, I've updated the Github repository to include these additions.