Your computer is designed to use a good chunk of the RAM in the background, to accelerate tasks that frankly do not need acceleration (because why not). I would expect, if you closed all your apps, you'd still see Windows (I assume this is Windows – Windows does it the most) using 16gb. How could it do this? How is Windows usable on computers with 4gb of RAM? Well it doesn't need to use all this RAM, it just does it because it can.

And the same thing goes in reverse. I'm frequently actually using 16gb of RAM or so, but the 8gb handles it fine. How? RAM Compression and Swapping. The Calendar app I have not used in the past 8 hours does not need to be using RAM, so it gets sent to swap (storage). The only way to notice this is that refocusing the app takes ~0.1 (~0.3 worst case) longer.

(Also, in my experience macOS and to a lesser extent linux is much better at compression than windows)