I saw a plane on YouTube that only had airlines or maybe there were elevants there was no elevator or rudder. How do you steer them without an elevator? They were towards the back so maybe they were elevants but they were pretty wide like the whole length of the wing pretty big wings it was a flying wing. How does the control system for that work? Are the aileron and elevator deflection voltages some and scaled together with some algorithm or op amp or voltage divider circuit before they are applied to the elevons?

Most transmitters have a feature referred to as wing type that allows you to set to " ailevator " this enables a mix so that when elevator is applied they both go up, when aileron is applied they go opposite. This would be a typical 2 servo arrangement. Prior to radio mixing we would have to use a 3 servo arrangement, one servo to drive an elevator located inboard and two servos or a belcrank system to drive ailerons out towards the wing tips. The Windfreak flying wing sailplane is a good example of the inboard elevators but uses rudder for roll control.

Another pre-mixing method was to mount an aileron servo in a sliding tray in the fuselage. The elevator servo would then push and pull the tray backward and forward, which would cause the ailerons to act as an elevator.

An in-between solution is the external mixer. After sliding trays but before computer radios you could buy an electronic mixer that plugged into the servo leads to mix elevators and ailerons. My last delta wing, a Balsa USA Force One was designed to use a central elevator and ailerons on the wing tips but was modified to use two control surfaces as elevons with computer mixing in the radio. I hadn't installed the pushrods yet but you can see the control surfaces on the trailing edge of the wing.