Clear android internal memory


I have a Moto G4 Plus (16 GB). Usable memory is around is 11 GB. After downloading a 9 GB of mkv files, I moved them to my PC. However, 2.5 GB in the internal memory is still occupied.

I deleted every folder, used apps like Master Clean & CCleaner. Nothing worked.

So, any idea how I can clear the internal memory?