The Sky commentators said VAR was going back to look at the approach play before confirming decision so I presume it was for DOGSO as they wouldn't check the APP for SFP. Unless there was a potential penalty or another potential red card beforehand as well of course!
I believe they were checking to see if the attacker was offside, if he was then a different start mode ie indirect free kick to arsenal would have occurred.
Personally i believe the red was issued for dogso but if it was sfp the red card should have stood if even if attacker offside. Pickford/van dijk/ oliver come to mnd.
If striker was offside and red card was for DOGSO then red card would be rescinded and play would restart with IDFK to Arsenal.
If striker was offside and red card was for SFP then VAR wouldn't intervene at all if red card correct - red card would stand and play would continue with free-kick to Liverpool.