How do I add a Samsung (HP) printer ExpressM2020W to my MacMini with OS 12.7.6?

No AirPrint, no drivers, and — somewhat unusually — the original vendor departed printing entirely, near as I can tell.

Time to either set up a Linux box as a pass-through print server, or replace.

Setting up a print server box is complex and somewhat fragile, and another thing to power and to manage.

Choosing a printer: https://discussions.apple.com/docs/DOC-250003486

AirPrint support for any Apple use, and IPP/IPPS support for mixed computing environments.

Brother is the local printing preference.

have you tried to download and install their driver?

Samsung Xpress SL-M2020W Laser Printer Software and Driver Downloads | HP® Support

when I choose macOS and version 12 it shows some stuff for download.

Unfortunately, HP appears to have removed Mac from the Select your Operating System menu, making it impossible to continue to Select your Operating System version and macOS 12 Monterey.

[This problem may or may not be limited to some HP local sites, and/or to the use of certain web browsers from some countries.]

Earlier, the latest Samsung universal driver offered on HP's (M2020) pages appears to have been V3.93.08 (Feb 27, 2023) for macOS 13 Ventura.

If necessary, try a community search.

You could perhaps check whether a link for something like the V3.93.08 or possibly an immediately preceding version is available from HP.

As always, make sure that you download from an official web site of a printer manufacturer only (Samsung printers now belong to HP). Do not download anything from dubious third-party "support" sites (often with misleading names) on the Internet.

Yes, the Samsung Xpress M2022W and M2026W are listed as supported AirPrint models, but not a plain M2020W.

About AirPrint - Apple Support

https://mfi.apple.com/account/airprint-search

In the user's guide, AirPrint is said to be a feature of the M202xW series (page 7), but (according to page 127) only AirPrint certified machines can use this feature.

https://h10032.www1.hp.com/ctg/Manual/c05790048.pdf

The OP could perhaps double-check this via the SyncThru™ Web Service (pp 149-150).

Unless one has been professionally billable (as I have), one often doesn't assign a cost to their wasted time. How many hours would one need to waste in opportunity cost to simply replace this ancient Samsung device, with a modern, AirPrint compatible Brother monochrome laser printer. Some choices.

No indication of this printer supporting AirPrint, either.

If it had supported AirPrint, you would have been able to use it without installing separate drivers.