Similar to the popular "what desktop OS".
In Germany, nearly everybody uses either a cheap router from their internet provider, or a FritzBox by AVM, running the proprietary FritzOS.
There is a ton of routers out there that are supported by OpenWRT, a free operating system for wireless routers, with a WebUI and a substantial amount of supported Software for it.
But those routers may be supported by OpenWRT, while not getting any firmware updates anymore.
There are a few manifacturers that produce routers with opensource software, like Turris with the Omnia.
Routers not only implement internet access, but often also DNS, a Firewall, IP/DNS blocklists, filesharing and more.
Those often overlap with the features of end devices like Smartphones and PCs. But still, one could say, better have 2 layers of security than 1.
What routers do you use?