EDIT/UPDATE: Recent versions of lemmy make this even easier by allowing turning the !instance format into a relative link to your instance. For example !div0@lemmy.dbzer0.com. However when you do this, don’t use the auto-fill option which turns it into a markdown link, as that will break this functionality.
When you link to another community on lemmy with a absolute link like so Div/0 this links to that community directly from that instance’s domain. This means users who follow that link from other instances, will not be able to comment or subscribe. They will have to go back and edit their URL manually to workaround this.
If you instead link to a community using a relative link like so Div/0 it will lead them to the community using their own instance as a mediator, allowing them to interact with it naturally.
The source for the above relative link is like this [Div/0](/c/div0@lemmy.dbzer0.com). So basically just add only the /c/... part and ensure your instance’s domain is at the end
Unfortunately I haven’t found a way to relativelink to specific posts yet as their IDs change from instance to instance.
Likewise, I haven’t found a way to relative link to another instance when putting it in a new posts’ URL