On piefed you can join :
- Peertube channel : https://tarte.nuage-libre.fr/topic/peertube/mondelibre and crosspost them.
- in late 2025 : follow mastodon account.
- follow Pixelfed : not supported.
- later in 2026, mobilizon : we can create event post. The base is ready for futur federation with mobilizon : https://codeberg.org/rimu/pyfedi/issues/291

On mbin, you can join :
- peertube channel
- mastodon account
- pixelfed (probably supported but i don't know well mbin)

Lemmy, i got bad result with peertube. Maybe over time they solved it, i haven't tried it. It was possible but you had to manually retrieve the video.

People from mastodon, pixelfed can post on lemmy, mbin, piefed