When you subscribe to a a community you can see all posts to that community. You can think of it as the post getting sent to whichever instance created the community, which then distributes the post to any instances that have users subscribed to that community. AFAIK there is no way to view all posts from some other instance other than visiting that instance directly.

You should be able to see all of the posts from communities that someone on your instance has subscribed to by switching to "All" in the community selector at the top of your feed (Subscribed / Local / All).

For discovering communities right now I am just going to the top instances listed on https://join-lemmy.org/instances and seeing what communities are active and interesting. The flow to subscribe to them kinda sucks (you have to copy the community name "!asklemmy@lemmy.ml" into the search on your instance, wait for it to do some sort of handshake, and eventually you can then click into and explore that specific community). Apparently there is also a community browser you can use.