ELI5 how do the posts we make on lemmy.ca show up for the rest of the world
That's the point of being federated. All the servers talk to each other and all the posts and other interactions are shared everywhere.
do we need to be posting on lemmy.world for that to happen?
No. lemmy.world is just another server (instance) like lemmy.ca. Those two and others talk to each other and share posts, votes, etc.
As in the title, is lemmy.world like r/all?
Nope. It's just another server (instance) like lemmy.ca
sorting out how to post beyond people on the lemmy.ca server.
All the posts you make on communities you access through lemmy.ca that are outside of lemmy.ca will be seen by everybody on every federated server that is looking at that community. Likewise, all posts you make on lemmy.ca communities will be seen by everybody on every federated server that is looking at that community.
This is true for all the posts everyone else makes everywhere else as well.
The most difficult thing for new users right now is how to find and subscribe to a community on a different server (instance). It's not hard, but you gotta know how to do it. The devs are working real hard to make this easy.