DIAGNOSIS: Baked-on grease and splatter on the microwave interior, especially on the mica cover that protects the magnetron waveguide.
FIX: For the interior walls and ceiling, use a microwave-safe bowl with 1 cup water and several tablespoons of white vinegar. Heat on high for 5 minutes to steam-loosen grime, then wipe. For the stubborn brown film on the cardboard-like panel inside the top wall (the mica cover), do NOT scrub it. This cover is fragile and critical for safety. If stained, it should be replaced, not cleaned aggressively.
IF THAT DIDN'T WORK: For grease on the stainless steel, use a dedicated stainless steel cleaner or a paste of baking soda and water. Avoid abrasive scrubs that will scratch the surface.
HEADS UP: Never remove or puncture the mica cover on the interior top wall. It's there to prevent grease and food from entering the magnetron cavity, which could cause arcing, damage, or fire.