The classic American aluminum muffin pan. Believe it or not the dirt doesn’t interfere with the baking. So if you don’t keep it clean every time, no food police will show up at your door. Keeping the cast off baking spray and other grease from building up, just makes it easier to clean going forward. But don’t try to OVER clean them, the aluminum isn’t that thick, and the aluminum oxide layer is only about 4 nanometers thick and it forms a protective coating. So clean but don’t try to sterilize it.
The first picture is your standard dirty muffin pan. The second pan, which I bought for a dollar had been cleaned with oven cleaner. It is not really worth the elbow grease to reclaim the finish, it still usable but kinda of ugly.
Sprinkling a baking pan with baking soda and ignore it in the sink. When you remember it’s there, scrub it with a green grill pad or if any of the spots are blackened use fine soap free steel wool.
Repeat the soaking overnight if needed. Clean any crevices or seams with a sharpened chop stick or plastic scraping tool; sharp metal tools will just scratch the aluminum. The same goes double for the muffin cup side, just sprinkle it with baking soda and a bit of water and leave it in the sink. No vinegar, no lemon juice, no chemicals. Aluminum is soft and reacts badly with any acids.
A more polished finish can be gotten using Bon Ami and a melamine sponge. More serious corrosion can be addressed with Bar-Keepers Friend, although it should be washed off immediately as it has a small amount of oxaclic acid in the recipe.