Yes, HDMI 2.1 is a specification that allows for up to 48gbps data. HDMI 2.0 allows for 18gbps.
Now, where it gets confusing is that some older passive HDMI cables are actually capable of carrying more than 18gbps of data... maybe even up to 48gbps. It really depends on the quality of copper that's in the cable. That said, you definitely want to have an Ultra High Speed (HDMI 2.1 Cable) if you're running longer runs... or if you're running a modern gaming rig to a TV with HDMI 2.1.
So that's one issue.
As for Atmos... if you're streaming a movie directly to your TV and dumping to a soundbar or a receiver capable of decoding Atmos, you can do that with a TV with HDMI 2.0 onboard. Yes, eARC is written into the HDMI 2.1 spec... but it can also be deployed under the HDMI 2.0 specification.
Confusing, I know.
The limiting factor will be whether or not your TV has received a firmware update that enables eARC. If it hasn't, then you're stuck with ARC. And, yes, if your TV only has ARC and you're streaming to an app on the television, then it won't dump Atmos out to your soundbar or receiver.
Does that make sense? If not, I can try explaining it in a different way!