depends on how deep into the rabbit hole you want to go.

The easiest way to build a router at home would be to use one of the free and open source software products like PFsense or opensense.

That will allow you to use an old computer or a specialty-made computer appliance with a couple of extra Ethernet ports and fully control its entire routing operations.

Many people use these because they are good and convenient and relatively cheap and not the most complicated thing to do.

Now if you're wanting to go a step higher, you could get something like the banana pi router with Wi-Fi 7 and 4 ethernet ports, and then install something like DDWRT or OpenWRT on it.

A lot more work, you need cases, you're going to be spending a good chunk of money for a very specific specialty appliance, and it's going to be 100% on you to support it as it's basically a non-standard system, so for almost every step along the way you'll be going blind.

Finally, if you're asking if you can just literally get some chips and some circuit boards and some Ethernet jacks and rig up a router, you're probably going to need more than a bachelor's degree in electrical engineering and a whole lot of time and money to figure out and fail and fail until you finally succeed.

It is possible, but it's not advisable.