RAM of MacBook Pro is always full

How much RAM do you have? macOS uses as much RAM as possible. If you open Activity Monitor in Applications /Utilities then click on Memory. Look at the Memory Pressure graph at the bottom of the Activity Monitor panel, if it is green you are fine, if it is yellow then keep an eye on it, if it is red then your Mac has a problem, perhaps a stray process

look at the list to see what app is using most of the RAM and quit it.

It is best not to run too many programmes/ apps at once as this will put a lot of pressure on your RAM.

Check that you don't have a lot of apps due to open when you start up the Mac.

The consensus here is that a Mac running ElCapitan or later needs 6GB or more of real RAM, or will be stuck in a paging RUT. More than that will be somewhat better, but getting above 6GB will improve things dramatically.

Cleaner apps are at cross-purposes for Macs running 10.9 or later, because the memory management approach has been completelychanged. The new mantra is "Unused RAM is wasted RAM" and now Memory Pressure (not free-memory), is the new metric to follow. (Green is good).

That item that says "file cache" will be freed instantly if an Application needs more RAM.

Using MacOS Mavericks or later?