find / -lname '/path/you/are/looking/for/*'

Note that the -lname option is a GNU find extension and may not work with other find implementations.

It asks for a path to a root directory of a bootstraped container. You can create it with debootstrap, rinse, pacstrap, alpine-chroot-install, virt-bootstrap etc.

virt-manager is able to work wit lxc. Add a new connection of type Libvirt-LXC.

Yes, you can.

Nope. Free beer ≠ free speech.

Do you really need OPNsense? Buying a OpenWrt capable router would save your money, place and silence.