Your analysis is correct. You have 2 mix busses which go out to line out 1,2 and out 3,4.
Your DAW may also display the FX bus so you can feed the UR44 onboard FX with signals from your DAW (as well as the inputs) and then mix them back into Mix 1 and Mix 2. Useful for Direct Monitoring with FX.
Headphone 2’s output can be set to either Mix 1 or Mix 2. Handy for a Cue Mix.
The integration of this with Cubase is tighter but you can achieve the same outcomes using the UR44 Mixer software along side your DAW.