Which aftermarket keyboards are compatible with a new Mac mini?

So many!

Even a keyboard with keys intended for a Windows PC will work with a Mac, but you'll have to learn to use a few different windows keys in place of the typical Mac keys.

Do a google search or a search on Amazon for Mac compatible keyboards and you'll find what you need. USB keyboards will work whether USB-C or USB-A types. Though a keyboard with a USB-A plug will require the use of a USB-A to USB-C plug adapter.

If you're looking for a wireless Bluetooth keyboard, again search for one that lists compatibility with Macs in its description.

Macally and Logitech are two brands that market Mac keyboards. There are many other off brands as well.

the HID interface is a generic interface

Human interface device - Wikipedia

so all usb keyboards should work

same with bluetooth keyboards List of Bluetooth profiles - Wikipedia

"Which aftermarket keyboards are compatible with a new Mac mini?: Which aftermarket keyboards will run with a new Mac mini"

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DuckDuckGo: define aftermarket

Vintage Keyboards Compatible with New Mac mini:

A wired keyboard(PS/2 or Serial) from the first decade (CRT iMacs) works just find on my 2002 iBook and 2007 MacBook. Never tried one(that I recall) on my 2010 and after Macs.

Ebay:

A. Mouses:

• vintage ps2 apple mouse
• vintage serial apple mouse

B. Keyboards:

• vintage serial apple mouse
• vintage serial apple keyboard