How to sign in when you see other urls/websites?

You are registered on lemmy.world - that's your instance.

The accidental renaissance community is separated into two distinct communities on two separate instances (not sure why they did this, instead of unifying into one): one on kbin.social, and one on lemmy.blahaj.zone. As they are separate, you probably want to subscribe to both.

You can access both communities using your lemmy.world account, but you need to edit the URL a bit to ensure you remain on your home instance (lemmy.world):

So instead of going to

https://lemmy.blahaj.zone/c/AccidentalRenaissance

for example, you need to instead head over to:

https://lemmy.world/c/accidentalrenaissance@lemmy.blahaj.zone

https://lemmy.world/c/AccidentalRenaissance@kbin.social

You'll notice that we append @the_name_of_the_instance_the_community_is_located_on to the end of the URL. Obviously, you don't need to do this for communities hosted on lemmy.world since... you're already here.

You can also link them in comments easily using an exclamation mark, e.g !accidentalrenaissance@lemmy.blahaj.zone and !accidentalrenaissance@kbin.social

There are ways to automatically ensure you're always on your home instance, so that this is less confusing. Personally, I use the Lemmy Universal Link Switcher script via the Tampermonkey browser extension. This ensures that all lemmy links are changed so they always lead to my home instance, lemmy.world.