I'm on KDE Neon 5.16.2 and as I've observed, when the systems boots up, it takes up around 400 MB of memory and if I just open and close different programs like VS Code, Firefox, KTorrent, KWrite, Libreoffice, OBS Studio, KDEnlive, etc., memory usage keeps going up. Seems like it never releases a portion of memory taken up by each of those programs unless I reboot the machine!

Why?

It likely is fine, as Linux uses ram differently, and different system monitors may show this info in different ways.

This page has a good explanation of why it seems like all the ram is 'not released':
https://www.linuxatemyram.com/

Basically, the application does release the ram, but the system keeps it around so it can be used for caching, etrc.. This is much quicker than fully releasing the ram, then having to access more of it later. Many system monitors don't account for this well.