Fast internet makes real-time interaction between musicians possible, under certain conditions over hundreds of kilometers and even across oceans. There are mostly private but also public live streaming sessions for those who wish.
A key element of long distance collaborations is a solid rhythm section, typically a high quality backing track. Live rhythm is possible but requires very low latency and very skilled musicians.
Currently there are participants from all over Europe and the East Coast of the US. Regional groups can be organized on demand. Participation is entirely free, only the host needs a paid subscription in JackTrip.
This group is exclusively for jazz with a strong emphasis on improvisation but, technically, other genres are possible as well.
Besides zero logistics online jams allow musicians to free them from local limitations where it can be hard to find like-minded people nearby.