Here's a few:
- https://meet.jit.si/ which you can also self host https://github.com/jitsi
- https://bigbluebutton.org/ which you can also self host https://github.com/bigbluebutton
I've found that they're especially useful, when integrated with Rocket.Chat https://rocket.chat/ which you can also self host https://github.com/RocketChatThat way you have an experience that's a lot like Slack/Teams, with pretty good support for chat, reactions, file uploads, discussions, making quotes etc., while also being able to start video/audio calls with the press of a single button.
Of course, if that's too many platforms, Rocket.Chat also supports WebRTC, albeit the UX was a bit less stellar when i last tried it.
Alternatively, there is also Nextcloud Talk, which can integrate with your instance of Nextcloud and allow for file sharing, chatting etc., though personally i found Rocket.Chat to be more usable: https://nextcloud.com/talk/
Regardless, those are some very competent options which allow all the data to remain on your own servers.