Switches are Layer-2 devices (data link layer). They operate on FRAMES and use MAC addresses to send data around between devices on the SAME NETWORK.

Routers are Layer-3 devices (network layer). They operate on PACKETS (which is basically a wrapper around FRAMES) and IP addresses to send traffic between DIFFERENT NETWORKS.

Switches may have some smart capabilities, such as creating separate logical networks (VLANs), or providing power to PoE devices, or prioritizing layer-2 traffic within a lan (CoS - class of service) and they do all the “heavy lifting” of slinging frames around to the right device on your LAN.

Routers tend to do all the “heavy lifting” of routing packets BETWEEN NETWORKS. They sit at the perimeter of networks (between your LAN and the internet, for example, or between your LAN and another DMZ LAN in your house, or maybe a GUEST LAN). They are often paired with firewall features to inspect the traffic and only allow certain types of traffic through one direction or the other, or they may simply route packets. They can also prioritize layer-3 traffic (QoS - quality of service).

A lot of things can get really confusing between the two because many routers have built-in switches, so they do some layer-2 stuff. And more expensive switches can even have some routing features to allow traffic to hop from one VLAN to another without going all the way out to a router (called layer-3 switches, though you typically don’t see these in homes outside the computer enthusiast community – they’re more of an enterprise thing).

I think the reason you don’t see OpenWRT or OPNSense for switches is because simple networks don’t need the advanced switching capabilities that such a product would provide, and highly complex networks often need the speed of hardware-based switching and don’t want to slow it down with a software layer.