use iMac 27 inch mid 2010 as a monitor, how?

None.

"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display."

From: Use your iMac as a display with target display mode - Apple Support

+1, sorry it will not work. Your MM needs an actual external display. Target Display Mode died in 2015 (6 years ago), the final nail in the coffin was Big Sur and the M1 chip.