What is a location of backend middleware??
I want to know where backend middleware like a can:media.medias.create is a located.i want to some changes in that middleware.
These are defined in the permissions.php file in the config directory of each Module. When the backend does a permissions check, it checks to see if that value in in the permissions list for the role of the user. I think it is just a pass/fail. If you want to inhance it I would make a new middleware and call it from controller or in the route definition depending on how you are using it.