In my experience, have flown 20 segments since lockdown, is that I choose my seat in FC or premium (as a individual traveler) then subsequently the adjacent seat is blocked.
Most using common sense would not”choose” a seat next to me when the majority of other seats are not occupied. Last Friday I was in 4F and apparently an upgrade was seated next to me in 4D. At gate the GA moved this person to 1F (all the D’s were empty on the flight).