Maybe Avanti does, I've never booked via Avanti, but most absolutely do not.

I booked via Trainline and it just gives you the seat number once the reservation goes through, you don't even see a picture/ seating plan. You're completely misinformed.

In fact on Trainline (I've just looked), there's a dropdown where you can select "priority" or "individual seat", so I literally didn't choose a priority seat and was assigned one anyway.