How to turn anything into a router

I don’t like to cover “current events” very much, but the American government just revealed a truly bewildering policy effectively banning import of new consumer router models. This is ridiculous for many reasons, but if this does indeed come to pass it may be beneficial to learn how to “homebrew” a router. Fortunately, you can make a router out of basically anything resembling a computer. I’ve used a linux powered mini-pc as my own router for many years, and have posted a few times before about how to make linux routers and firewalls in that time.