Someone posted this earlier and it’s a good explanation

https://www.linuxatemyram.com/

Here is an Ug level explanation if you want something shorter:

Disk slow, memory fast (even compared to an SSD)

If memory fast, why no use memory

Some things load from disk often but slow to wait. Load from disk in advance and keep in memory so fast. Now use memory better.

If not enough memory we let go of other thing to run program.

How work? Magic.

Use something like ps to list out what is using the RAM

You mention you closed Firefox and it didn’t change but you did not say how you actually measured it.

Start the system up fresh and run this command.

ps faux --sort=-%mem

Or

ps faux --sort=-rss

That will help you track down what is actually using it.

Note: browsers will soak up a ton of memory on modern systems as almost all of them sandbox tabs at this point. If you open a ton of tabs, that will be half your problem.

That saying is because “unused RAM” is RAM that is not being used for buffers or cache.

That RAM is quickly freed for applications when it’s requested.