Is it possible to create an API inside an iOS app to let 3rd party developers create app add-on that are downloadable for users inside the app?
I could not find any ressources for this topic online.
I think that's going to go against the app store guidelines if your intention is that the 3rd party developers write code, which will be downloaded and executed.
There's something of a grey area between legitimate uses and illegitimate uses of downloaded code:
Legitimate Example 1: Something like Hopscotch where children are creating simple fun shared games. These can be considered user generated content.
Legitimate Example 2: A game which is driven by scripts allows for the run-time downloading of patches to fix bugs in the scripts or even to add new levels. I believe such usages have been rejected occasionally in the past, but are generally accepted these days.
The relevant guideline from the guidelines is (emphasis mine):
2.5.2 Apps should be self-contained in their bundles, and may not read or write data outside the designated container area, nor may they download, install, or execute code, including other iOS, watchOS, Mac OS X, or tvOS apps.
And the all-encompassing:
We will reject apps for any content or behavior that we believe is over the line. What line, you ask? Well, as a Supreme Court Justice once said, "I'll know it when I see it". And we think that you will also know it when you cross it.
If your idea is to create some sort of app-store within your app-store app, then I would abandon it immediately, because that's going to be way over Apple's line.
That said, you haven't given much detail about your app, so I might have made wrong assumptions about where you're thinking of going.