I have a J5 Create USB-C to 4 Port HDMI adapter. When I updated to Big Sur my external monitors were not recognized. Usually, when this happens I would unplug the adapter and replug it in and the iMac display flashes, then the external monitors come on. This time, nothing. I called apple. They were trying, but because the adapter isn't apple, they can only help you a little. SOLUTION: Update your drivers!!!!!!! Apparently, the driver for the adapter was for Catalina. Since I upgraded to Big Sur, I needed to update the driver also, then restart cpu. Once it loaded up, I had to go into SYSTEM PREFERENCES---->Extensions. I saw that I had to allow the driver before it was ready. Then one more restart and bam! My external displays are now working properly. I really hope this helps someone as I was irate!!!