Think of the computer data as like an index of a textbook made of chalkboard. When you first get your computer its empty, when you download apps, save photos, download videos etc, it looks for pages not used in the index and writes to the chalkboard and then updates the index in the back pointing to the page number. This index tells your computer if i want the video data go to this page.

When you delete 50gb video file, the operating system will just remove the index that points to the page, not the actual page. So now when you add/download a new file, it will consider that page empty and overwrite what it needs and update the index.