I think a better analogy would be swapping tires instead of an engine. Engine swaps are rare, most people understand there are physical complications to using an engine from one car in another, even from the same manufacturer. Tire swaps are common, most people expect that if the tire is the right size you can put it on your car, regardless of who made it or where you got it.

The same is true for apps on a computing device. Installing new ones is common, not rare like an engine swap in a car. Most people expect that if the app is compatible with the hardware you can put it on the device, regardless of who made it or where you got it.