It really depends on what you are trying to do. If you have an ip set in network-manager and then you set the same ip in the OMV web interface, obviously it will keep doing the right thing but both will be doing it. If you have it set to dhcp, it is possible you could get two dhcp address.
OMV uses netplan and systemd-networkd.
Neither knows nor cares about the other one. It is possible to control network-manager with netplan but OMV does not.
Somewhat. They are both affecting the same network adapters but that is where it ends.
You really don't want to continue to use both. Pick one.
Thanks for answering each of my questions. It makes a lot more sense now on how with network-manager installed, there would effectively be two systems controlling the same network adapters. The whole thing is fascinating to me as I get more familiar with Linux, learning the ropes and try to setup the "ultimate system", currently with OMV running the show so appreciate you clarifying things.
I looked up Netplan and was interested to see that it supports both Systemd-networkd and NetworkManager as "renderers". My brain can't help but wonder if the Netplan files OMV generates/uses (abstracted network settings in YAML format from the look of it?) can be "applied" to network-manager. I suppose I've answered my own question - they can't since then it could/would overwrite settings that Home Assistant needs. Just thinking out loud and something that's clearly a pointless pursuit in practice. I certainly don't want to take up any more of anyone's time with this as the answers so far are clear. Sometimes I need to know when to stop! As mentioned, I'll now almost certainly keep Home Assistant on my Pi4.