What is inside an oil filler/breather cap?

I’ve done this and the mesh always comes apart and if you shake it the mesh will fall out of it. So now I don’t want to use it on the info e any more. So I don’t clean them now.
What was the mesh made of? When I look in the bottom, I see a (looks like) metal mesh screen with 1/4" square holes. Don't seen anything when I look in the air cleaner side tube.