Disk Space
Why is my hard disk space reducing after watching online videos? and how to prevent this?
MacBook Pro 15″, macOS 10.15
One possibility,
it matters not that you were watching videos "online," all of the data was downloaded to your computer in order for you to view it.
If you have an SSD, the space is not immediately released when you delete something. The drive has to reset the the memory locations to make them usable, again. That happens when it runs its garbage collection routine, generally overnight when it is not in active use.
Even if you have a HDD, the data is stored in temp storage and will get cleared after some time has passed.