Multi-monitor use with Mac mini (M1, 2020) on macOS Big Sur 11.2.3.

+1 You were given bad info by that Apple Retailer.

Please review the M1 spec’s at > https://www.apple.com/mac-mini/specs/

Plus connecting displays at > https://support.apple.com/guide/mac-mini/connect-a-display-apd8e4fbbb97/mac

I have a 16 GB M1 Mac mini driving three monitors in a flight simulator as follows: 

X-Plane 11.55.2 on a 16 GB M1 Mac Mini connected

1. To a Dell model 2007 FPS monitor (1600 x 1200 @ 60 Hz via VGA on monitor to VGA/DVI-I adapter to Startech.com USB 3.0 Dual Head Graphics Adapter to USB-C on Mac Mini), and
2. To a Westinghouse TV model LDD2240 (1920 x 1080 @ 60 Hz via HDMI to HDMI), and
3. To a Flatron W2252TQ (VGA ơn monitor to Belkin VGA/Thunderbolt adapter to Thunderbolt on Mac Mini)

DisplayLink driver from Startech.com was loaded and activated. OS is Big Sur 11.4.  Logitech Saitek Yoke, Power Quadrant, Rudder Pedals, and Radio Stack.  Flight Velocity Pro Trim Wheel.  Mini iPad with ForeFlight is mounted on the yoke and provides GPS position on a map (VFR, IFR, or Satellite).  Another mini iPad with Air Manager V.3.7 from Sim Innovations shows the instrument panel, and allows one to have only external views on the three monitors. 

An article with full description of this flight simulator can be found in on pages 50-53 of the August/September issue of ‘Midwest Flyer’.  The title of the article is “Fighting Lock-down with a Flight Simulator’.  If search via that title, you can find a text only version. The full .pdf of that issue shows photos of the three monitor system. 

The M1 mini doesn’t support 3 monitors, only two, one of which must be HDMI. I would return the entire lot to BS Buy and re-buy at an Apple Store/online. Good luck!

This link may take you to the full article mentioned above:

http://drive.google.com/file/d/1ID7FhV-RvzKlrEUSrur1hr2EC9R1tADm/view