Help me delete the other files(samsung M35)

I don't know the other files keep building up i think my files are backing up somewhere but i dont have onedrive or google drive signed in(i don't even know how to backup files) but when i delte movies or something the memory doesn't free up i don't know what to do Like once the other files.was upto 80 gb i was really worried about my storage i did everythimg but nothing happend but i woke up in the morning and all the other files were deleted i thought i did something but it was auto optimisation feature this according to me...Please help how do i regulate my data storage or will auto optimisatiom would do it for me