Whenever I've asked people about how to send out a beta through email, they've always told me "Oh, it's easy. You just export the IPA, then send everyone the link." I've done this myself as a tester. I tap the link, and it installs. Easy as that.
And now I can't find out how to do that.
For me, I export, and attach the link in an email to myself, and it forwards me to safari, with an "Open with..." prompt. I can't figure out how to get it to install.
I'm not interested in TestFlight or anything that involves more complication. I just want to send people my build, and can't figure out which part I'm getting wrong.
Assuming (and I hope you don't) jailbreak targeted devices:
With a basic Apple developer licence, you'll need to add devices UDID into the provisioning profile you built your app with. Using a third-party service is a great solution (Crashlytics, TestFlight).
If you have an Enterprise Apple Developer plan, you can build your app with your certificate and install it on a device without adding its UDID before - that's what you need. Once you built the app, you'll just have to host the generated .ipa on a web-server with HTTPS and send the direct link to your customer. Please find the official documentation here and here.