I am currently using Lemmy.zip through the app voyager.
I can also see content from other places like hexbear and fedinsfw.
If I login to lemmy.zip on a desktop I can use my voyager credentials. But I am only able to view lemmy.world and nothing else here.
If I login to fedinsfw on desktop I can seemingly access a multitude of places such as I can on voyager, including lemmy.zip.
I’m sorry if this is such a dumb thing to get confused about. I’ve heard explanation for what all this is, but I’m still just not quite sure I understand.
In addition to what others have said, your server protects you from other servers.
Only your home instance has your login credentials. When you “go” to another server through yours, you aren’t actually going anywhere. Your server fetches it for you. No other server receives any identifying info about you. So you don’t need to vet every server you can access content from.