You can share content with users of other instances, or import content that is being shared with you into your own account. This is federation, but still quite limited.

Example: I can share a folder from I@myserver.org with you@yourserver.net

Syncthing is equivalent to the file-sync portion of Nextcloud and really is p2p FWIW. Been happy enough to replace my Onedrive subscription and once set up, just works.

It's not peer-to-peer, but you can share files between Nextcloud instances.