

The most useful is probably cat /proc/meminfo. The first couple of lines tell you everything you need to know.
MemTotalis the total useful memory.MemFreeis how much memory is not used by anything.Cachedis memory used by various caches,e.g. ZFS. This memory can be reallocated.MemAvailableis how much memory can be allocated, i.e.MemFree + Cached.
You should also look at which processes use the largest amount of memory. ZFS is weird and might allocate its cache memory as “used” instead of “cached”. See here to set its limits: https://forum.proxmox.com/threads/limit-zfs-memory.140803/