I can verify myself using my Id card and a terminal or via NFC. The app used is the middleware between software or website requesting verification and my ID. No data leaves my ID in direction to the software or website. The middleware just gives a thumbs up to the verification request.
Well, you CAN participate, subscribe and read the community of another instance. You don’t have to be part of the other instance to do so.
Just make sure you are browsing / searching all and not just local.
I am reading and writing this from kbin and not even Lemmy btw.