You have multiple options to distribute your builds to testers, you do not have the option to specifically tie this to pull request events, but you could configure a trigger to run a workflow that does this on pull requests. The pre-built solution is our Deploy to Bitrise.io step, which creates a public install page for your IPA/APK that you can forward to any address.
As for your second question, the option to register iOS test devices is there because it’s necessary because you wouldn’t be able to install artifacts on your device due to Apple’s restrictions, but the same isn’t true for Android. Can you let us know why would you like to register such devices or what do you want to accomplish by it?