Why is OPNsense, pfSense, etc an entire operating system? Do I really need to "install" it on bare metal?

Hello friends,

I am considering getting into this stuff, but on both websites the "get started" pages discuss creating a bootable media device to then install the software to a target storage device.

I am confused because, well, from my limited understanding of things, I don't see why it can't just be a program within an existing linux/windows OS. It seems like I'll be made to run it within a vm, container, or whatever of that sort.

I've seen some mentions of virtualization / virtual environments on both sites installation pages. But that raises concerns - that it may become marginally more difficult to install / setup, and concerns of potential performance issues (throughput & latency).

My GOAL is to use an old DDR4 system, install whatever light Linux distro, install whatever NIC, and use it as my general home server. For hosting game servers, websites, my NAS RAID, etc.

So I... might assume... if the moden plugs directly to this machine, it then wires into the virtual machine running OPNsense... and then the host OS connects to the internet through some kind of virtual ethernet connection between the host OS and the virtual OPNsense router. Just sounds... quite a bit complicated.

Hopefully I made it clear what I'm worried about.