This is a tricky question! If you have created something then you own it, but the app/game you made it on may also have permission to use or change it however they wish. This is because you used their app/tools to create and share your work.
When you create an account on an app/game, you agree to the terms and conditions (the ‘rules’) which might also include rules about who owns the things you create on that platform.
Unless you have given permission, other people online are not allowed to use your work.