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 certa

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.