UPDATE 7 FEBRUARY 2023
A macOS update apparently made the version 2.0 drivers stop working. I also ran into an issue where, because I'm using custom display profile overrides the tablet settings app cannot find the names of my displays nor could it tell there were two of them (it sees both displays as one ultrawide). After some playing around with both newer and older drivers I am now recommending you use the version 2.1.6 driver. It does have a bug that causes it to sometimes lose all your custom settings (like hotkey assignments), which you can mitigate by locking the actual settings file so it's "read only." (You can find it at ~/Library/Preferences/com.ugee.pentabletsetting.plist and lock it by right+clicking and select "Get Info" then in that dialog tick the "Lock" box) But don't do that until after you've finished getting everything set up the way you like. Also, some apps simply won't accept your custom settings from this driver. For example, even with the default button assignments on my tablet where K1 = increase brush size, K2 = decrease brush size, CSP sees the first one correctly - but inconsistently - and the second one as "eye dropper." Even if I assign those to "]" and "[" it shows at the bottom of the display that the tablet IS using the configured keyboard character, but CSP ignores it. In Affinity Photo, though, it all works as intended. So this comes with a heavy dose of YMMV. You can still try the version 2.0 drivers and see if they work for you, but otherwise:
DOWNLOAD THE XP-PEN 2.1.6 DRIVERS FOR macOS
The 2.1.6 drivers *should* automatically create entries for the macOS Privacy settings as shown below in the original post, but you'll want to make sure they are checked, and your tablet will not work until you restart your Mac. On restart you'll likely see a bunch of privacy/security pop-ups and you need to authorize all of them.
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Both I and one of my friends own the same model XP-PEN Artist 15.6 display tablet and M1 Macs. It's a nice tablet for the money but the support for Mac users is terrible, especially since the M1 "Apple Silicon" chip was introduced. The tablet itself is actually made by Ugee and I think XP-PEN is just one of many "brands" the same tablets are sold under (their websites have the same layout, the driver files are the same versions, the interface for their preferences app is the same, they just swap out some images).
So, anyway, the "New UI" version 3.x drivers are garbage. They absolutely do NOT work. Version 3.0.5 would at least detect the tablet if you unplugged it and plugged it back in, though you couldn't configure the pen or the Express Keys as none of the sections would appear in the left-hand column of the preferences app. The versions after that won't detect the tablet at all.
I'm using the USB-C to USB-A + HDMI cable that came with my tablet because I'm mirroring my primary display to the tablet through an HDMI splitter. I haven't tried connecting my display tablet with a USB-C cable, but from what I've read on their forums and Reddit, it doesn't sound like that would make any difference.
The (as of this writing) current 2.1.8.2 drivers are listed as the "Official" ones on XP-PEN's website, and if you have an M1 Mac they will get you up and running, but you won't be able to assign your Express Keys. There's a bug in those drivers that won't let you reassign them properly and I doubt they'll ever fix it because they've moved on to developing their worthless 3.x drivers.
The Express Keys bug was also present in 2.1.7 version of the drivers and a different bug was present in the 2.1.6 drivers. The 2.1.6 will let you reassign the functions of the keys, but they were inconsistent in actually working. When yo press one it displays a message across the bottom of the screen confirming not only that the button was pushed by tells you in bright green lettering what it's supposed to do. Except on a M1 Mac it wouldn't always actually do it (the drivers seem to work fine on my Intel Mac).
XP-PEN frustratingly does not provide an archive of older versions of their drivers. And, no, you can't get them through the Internet Archive Wayback Machine either. Thankfully between three computers I had installers for several different older versions.
After installing and testing and uninstalling ALL of them on my M1 Mac I finally found ONE that actually works the way it's supposed to!
The Version 2.0 drivers that ACTUALLY WORK are, ironically, the oldest ones I have. The ones I downloaded when I bought my tablet.
You can get them from my Google Drive but installing them in Big Sur is not exactly straight forward.
1. UNINSTALL your current XP-PEN drivers! Use the "Uninstall" app in the same folder as the Settings app.
2. RESTART your computer to make sure it's gone.
3. Download the installer ZIP from that link in the line above.
4. Unzip it and inside there should be one file: XP-Pen Mac(20180425).pkg
5. Right+Click on that file so the context menu shows up.
6. Press the OPTION key on your keyboard and then select "Open" from the context menu.
7. The macOS Gatekeeper will warn you about opening it, go ahead and do it anyway.
8. Run the installer and wait for it to finish.
9. It will finish saying "Installation Failed!" but it's a liar, the installation worked just fine. Close the installer.
10. Go to Applications --> PenTablet --> PenTabletSetting.app and open it.
11. Authorize the app in any macOS security and privacy pop-ups you see.
12. Open System Preferences and go to Security & Privacy
14. Scroll the left column down to "Accessibility"
15. Click the [+] button under the list on the right and navigate to:
Applications --> PenTablet --> PenTabletSetting.app
16. Click the [+] button again and this time navigate to:
Macintosh HD --> Library --> Application Support --> PenTablet --> PenTabletDriver.app
17. Now scroll the list on the left to "Input Monitoring" (and we're going to do the same...)
18. Click the [+] button under the list on the right and navigate to:
Applications --> PenTablet --> PenTabletSetting.app
19. Click the [+] button again and this time navigate to:
Macintosh HD --> Library --> Application Support --> PenTablet --> PenTabletDriver.app
20. Restart your M1 Mac.
21. Plug in your XP-PEN display tablet.
22. Go to Applications --> PenTablet --> PenTabletSetting.app and you should be able to set your preferences for pen buttons, pressure, display mapping, and Express Key assignment.
23. EXIT the settings app.
Your settings should now be usable with your tablet. When you press one of the Express Keys on the tablet you should see a gray bar across the bottom of the screen with text telling you what action is assigned to that button.
When you re-open the settings app be aware you may need to select your tablet from the drop-down list at the top before it will actually load your custom settings. I had made my pen pressure softer but when I reopened the settings app it wasn't showing the curve I'd set for the pen, but after selecting "Artist 15.6" from the list at the top it loaded my settings. They seem to be persistent after that, but if they revert to showing defaults I'd try selecting the tablet in the drop-down first, just to see if your custom settings load.
This WILL fix your issue assigning the Express Keys custom functions. Whether using these old drivers ends up causing you other problems I can't say.
