How to find out which instance is federated with which instance?

Each instance of Lemmy has their own list, like so: https://feddit.org/instances https://feddit.nl/instances

Every instances has a page with linked and blocked instances: https://feddit.nl/instances

In summary

• Most of the instances are federated with each other
• One outstanding group is hexbear.net and lemmygrad, that quite a few people defederate
• Beehaw.org defederated Lemmy.world and sh.itjust.works
• Lemmynsfw is usually defederated due to its content being mostly porn

Defed Investigator seems pretty useful

Scroll down to the bottom of the desktop page and you will see the instances tab that shows the linked instances.