That is USED memory, it’s not including cached memory in that calculation.
You say you “only” have … & Firefox & … but Firefox, like any browser can use 100s of GBs of memory depending on the number and complexity of the tabs you have open, let alone whatever you’re running in VS code.
It’s like me saying “I’ve read every book in the Library” without defining whether I’m talking about a library with one childs colouring book or the Library of Alexandria the day before it burned down.