By newer macbooks, its just macbooks newer than 2015. The 2015s and earlier didn't have h265 encoders in hardware, and thus don't work properly. There is a hack you can install to force sidecar to run on earlier macs, but the latency and quality is terrible, partially because h264 is harder to shove over the wire than more compressed h256

Yes, I think it relies on some features of a recent-ish Bluetooth version.

To be fair, I never use it for something requiring complex input, but it’s nice for something like a movie, a music app, or something that runs in the background but you might want to have a glance at every now and then.