I’ve had a microwave for over 20 years. Microwave ovens do get awfully dirty. It can be a chore to clean off the stuck-on foods.
Prevention is, of course, the best. Cover bowls with cling film or a paper towel/napkin to keep spills and splatters to the minimum. I try to wipe out the microwave after each use with a damp cloth. That’s the easy way - keep it clean to begin with. But, when an entire family shares the microwave but does not share the cleanup, well it gets pretty grimey, doesn’t it?
My favorite tip came from TV. Place one to two cups of water in a microwave-safe glass container. The container should have handles on it to make it easy to remove. Turn the microwave on the highest setting and allow the water to boil in the microwave for five minutes or longer. Carefully remove the cups of hot water with a towel or pot pad. Then wipe out the microwave with a damp cloth.
Yes, you can use mild detergents. I like to use a mild abrasive occassionally - something like baking soda (if that is available in your part of the world.) You don’t want to use any chemical cleaners, of course, since you put food in the microwave.