Good to know, thanks!

Ah that must be it sorry. I thought they had decorelated phone numbers and IDs

Groups have an encryption key that I guess you receive from other members upon joining.

Spaces is an underused feature that I hope see gain more traction! It makes Matrix a credible competitor to Slack and Discord

Not really, have used it for years like that. But you need to set it up initially on your phone. The newish feature (less than a year) is that I think they do not require a phone number to set up a new account.