There are a couple of things I’ve done to remove smoke and other debris from inside a microwave. First put a mug of water in there and get it boiling. After the microwave turns off, let it sit while the steam softens up the gunk on the walls. Then use baking soda as a scouring powder.
Baking soda will NEVER scratch because it dissolves in water, and you can use it to detail lots of other things, such as toasters and those buttons on the front of a dishwasher.
I’ve also had success by first spraying the microwave with window cleaner, then steaming it, and wiping it down after everything is softened. Use an old toothbrush for the nooks and crannies. You can also use baking soda and ammonia together with no problems.