For Teams-like chat I really like Zulip. Which also integrates with Jitsi for video conferencing

If you are hosting webinars there's also bigbluebutton

It's not open source, but up until a few years ago I used whereby.com for videochats.

Unlike the alternatives at the time from Google, Apple, etc., it didn't require an account for participants — I could just give them the meeting room URL. So although it wasn't open source, it at least didn't lock you into a network.

(Unlike you, I wasn't up for self-hosting.)

This one is often overlook but very good, I prefer it over Jitsi https://galene.org/

The French government built their own: https://github.com/suitenumerique/meet

Jitsi? https://github.com/jitsi/jitsi-meet

I guess Jitsi?

https://www.rocket.chat/