Can I use a rim brake rear wheel in a disc brake bike, for indoor training? I have a disc road bike (QR); I use it also with an indoor trainer with a specific training tire.
I would like to buy a wheel to be used only on the trainer (with the specific tire).

Since I don't use the brakes on the indoor trainer (no descents there...) could I use a non-disc wheel on my bike for the indoor trainer? They would cost less and easily found on 2nd hand market...

Or are disc wheels actually different (both QR obviously)?
The standard hub width for road wheels with QR is 130mm for non disc and 135mm for disc hubs. Even though you can force the frame to clamp around narrower hub, it is strongly recommended to add spacers to the axle to adjust the width. As mentioned in comments, if you have hydraulic brake it's best to insert a spacer between brake pads so that you do not eject the pistons if you accidentally grab the brake.