I have the same problem. iMac on Monterrey (2017) and MacBook (2020) on big sur.
ucb-C thunderbolt 3 connector between the two does not work. Called apple and was told that this is a known issues with the latest (post 2016) iMacs. And that it’s due to the newer software. No known fix that they’re aware of.
Very frustrating considering the ucb-c thunderbolt 3 was advertised as compatible which I took to mean that it would do its job and allow for dual monitor use between iMac-MAcBook pro.
selling the iMac at this point.
Unixer
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