The DRIGERA previously supported the OTBR REST API for credentials management, but unfortunately IKEA recently “patched” that, making it substantially more difficult to join a new DRIGERA to an existing HAOS+OTBR mesh.
The DRIGERA now supports the new Thread v1.4 credential sharing feature, but unfortunately HAOS+OTBR does not — yet. (if you’re running OTBR in your own docker container then you have more flexibility to update for this feature). I believe, but haven’t confirmed, it uses the ot-ctl ba ephemeralkey command to activate a one-time passcode for DRIGERA to join your mesh.
HomePod does not support either management feature, so most Apple Home users end up configuring their HAOS+OTBR to use the credentials from Apple Home, rather than the other way around. I’ve read reports that if you use the iPhone Home Assistant companion app to “Send credentials to phone” before you setup HomePod, that it will grab those credentials from your iOS keychain when you do the HomePod setup. But if you’ve already setup HomePod, you probably want to avoid adding a second credentials dataset to your iOS keychain because that tends to complicate (break?) commissioning new devices.