For Linux this will vary between distributions and configurations. For example, based on some testing I did today using mkosi [1] (for reasons unrelated to this discussion), a bare-bones Fedora 43 installation uses about 130 MiB of RAM, while a Debian installation uses a little more than 100 MiB.

IIRC last time I tried a bare-bones FreeBSD installation it used about the same amount of memory, maybe a little more based on how ZFS is set up.

[1]: https://github.com/systemd/mkosi

ZFS will happilly (and intentionally) gobble up available RAM for ARC. On my 64GB system, ARC is using 42.4GB, but this memory is quickly reclaimable if it's needed. That said, I had very bad experiences trying to run ZFS on an underprovisioned system.