/c/community@instance and @community@instance will work for communities.
/u/user@instance is instance agnostic but will not generate a mention to the user’s inbox.
[linktext](http://instance/u/user) will link to the user’s instance only, but it sends a mention to them. You can also do this by starting to type @user@instance and selecting the user from the dropdown box (on the website, not necessarily in apps).
The different apps for lemmy don’t necessarily always work correctly in line with the website, but they’ll catch up.
Each post is actually primarily hosted at that user’s instance, then federated from there. So if you’re a lemmy.world user and post in a lemmy.ml community, your post will actually post at lemmy.world first and then federate. It isn’t necessarily hosted in the community’s instance. The same is true for every comment - hence why you won’t see comments from users in an instance yours isn’t federated with, even if they are federated with the post’s instance and can comment in the thread.
You can’t really do anything with posts or comments to jump across instances, as each instance assigns a different number to the same content. Hopefully one day they’ll change it so the url is more like http://instance/post/123456@hostinstance in everywhere but the federated host instance. But it’s all still in early development.