This question already has an answer here:
When I'm creating drawables some of them goes to v21, v24 folder.
Why is this happening?
Will they appear in andoid api 15 (android 4)?
Do I need to do something?
Those drawable folders are actually for providing device compatibility (i.e. to manage different screen densities in android) and different android versions.
来源：https://stackoverflow.com/questions/47093515/drawable-v21-v24-what-is-it