I'm referring to what Activity Monitor app tells me in its memory tab - not the underlying malloc/whatever implementation being used.

It tells me my computer is using 8gb of RAM after a restart and I haven't begun to open or close anything.

Yikes?

Activity Monitor counts everything from I/O buffer caches to frame buffers in the "memory used" metric. Add in that MacOS won't free pages until it hits a certain memory pressure and you get a high usage with no desktop apps open.

This also means that cleanly booted machine with 16 GB will show more memory used than a machine with 8 GB.

Apple suggests you use the memory pressure graph instead to determine whether you're low on memory for this reason.

It's very hard to measure memory use because it's reactive to how much RAM you have; if you have more then it's going to use it. That doesn't necessarily mean there are any issues.