Ok so lets say i get the DJI FPV drone with goggles.......and i also would like to build my own FPV drone one day with DJI equipment (camera etc) I assume i would be able to use the goggles that came withe the FPV drone on the one i build.......but can i also use the Controller that came with the FPV drone on the one i built?  Or do i have to get a seperate controller?

You can use the Goggles with the Air Unit and the Caddex unit, currently the FPV controller that comes with the combo can only be used with the FPV drone.

The current Air unit has been discontinued, the Caddex is still available. There is a rumour that there maybe an Air unit V2 mid April, but it’s not confirmed nor is it’s compatible with the FPV controller.

Currently the only remote that works with the Air/Caddex is the DJI DigitalFPV system one.

Hi, Ikeman01. Thanks for reaching out and posting your inquiry. Currently, the DJI FPV Remote Controller 2 is only compatible with the DJI FPV Drone.

I'd like to find out if DJI plans to fix the issue where in order to switch from fpv to air units re linking is necessary, and you cannot have more than one "conventional" fpv drone linked as you can on the V1 goggles, to me this is a disaster from DJI  a $600 dollar disaster

Yes you can, when in Digital FPV Mode, you can have more
than one Air/Caddex unit paired with the goggles & FPV remote.

I have 4 air units paired with my V2 goggles.

The only issue I have is if you switch back to FPV Drone and repair with it you lose the all the other pairings.

A fix apparently is coming for that.