The Screen Sharing works well! Thanks a lot!The Late 2009 - Mid 2010 27" iMacs were the only ones that could be used as displays by Target Display Mode for non-Apple devices, such as a Windows PC.
The 21" and 27" Mid 2011 to the 27" Late 2013 and 21" 2014 iMacs could be used as a display by other Macs with Thunderbolt 1, 2 , or 3, via Target Display Mode, but not the M1 Macs.
Not sure why Apple did not add Target Display Mode connectivity to the M1 Macs.
Other than that, there is no HW solutions to use them as a Display.
If your "new" MBP is a M1, you could try a software solution, such as Luna Display.
A free option is Screen Sharing.
Screen Sharing is built into the OS, but there is a tradeoff between quality and low latency. You can one, or the other, but not both. I use Screen Sharing all the time, basically control 4+ Macs from one central Mac, and some of the Macs that I Screen Share with are headless.
Screen Sharing works very well depending on what you need from it. It works much better you can have a wired connection.
My mac book (m1 2021) doesn't find my old 2011 imac. I tried both with screen mirroring and with the thunderbolt cable. Does anyone know a solution?