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You may find confusing how there could be at the same communities (subreddits) with the same name in different instances. You can use the Lemmy Community Browser to find communities easily among different instances.
You're posts are visible from other Fediverse sites, like Mastodon or Kbin and you can interact with their users.
An user or an instance can block users or instances. If you have an account on lemmy.world and the instance admins blocks another instance you won't be able to see the posts or comments made from their users.
Is there anything in particular that you want to know?
To add to this great advice: lemmyverse.net. Fantastic way to find your favourite - or totally new - communities.
Thanks for the link! Was wondering how to look for new communities without having to search in the communities search option on lemmy.world itself