One thing you need to know is that communities of other Lemmy servers (instances) are not “visible” to your instance automatically (there is a Github issue in the Lemmy repo that attempts to solve this in the v1.1.0 release), so you, as a user, have to introduce it to your instance.

This is only required if the instance has never seen the community before.

You can read how you can accomplish this in the Lemmy docs or the Fedecan guide:

• Lemmy docs: https://join-lemmy.org/docs/administration/federation_getting_started.html
• Fedecan guide: https://fedecan.ca/en/guide/lemmy/for-users/how-to-open-in-my-instance

The Fedecan guide might also help you understand the Fediverse and how Lemmy works better. Check the whole guide: https://fedecan.ca/en/guide/lemmy/overview

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The second thing you should know is that the whole thing about every instance being federated with every other instances is a big exxageration.

Many instances defederate with each other.

Defederation means your instance won’t “communicate” with the defederated instance. This means your interactions will not be visible to the other instance with one exception, and that is when you are posting to a remote community on a instance that is not defederated with your instance AND is not defederated with the instance that your instance defederated with.

Communities in Lemmy are built on ActivityPub Group actor types and the specification states that the instance of the group actor announces activities, not your instance: https://fediverse.codeberg.page/fep/fep/1b12/

That basically means the instance of the community acts like a middle man that delivers messages.

In this case, the defederated instance will be able to interact with your posts in that community while your instance never will know about it.