Jammr is a delayed loop of limited size.
It is not real-time.
You play behind what someone else has done and feels a bit weird to do.

Zoom is real-time but only one person can play at a time.

Jamkazam plaudits seem to be scripted and I have not yet known anybody who has used it.

You can, of course, play along to any track you select on utube but it is not the same buzz as live playing.