On most airlines you can book the adjacent seat - technically for people with musical instruments like cellos or for “passengers of size”, most will let you keep the seat. If the plane is full and they’re trying to get a friend on board they might try to take it away. Tell them you’re worried about covid.

On United, I believe you book the ticket under the name ExtraSeat Lastname.

If you’re traveling with a cello, Cello Lastname works on a lot of airlines.

More complicated for international flights, but domestic is fine.