MacBook Pro 2021: How to Use Your Old iMac as a Monitor

Your 2011 iMac was capable of being a monitor for certain other Macs, but not your Macbook Pro.The article zKaczor links says this:

• • The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.

Therefore your MacBook Pro is too new. A free-standing external monitor uses cheaper cabling, is easier to set up, uses far less power and, important to me, allows extended desktop. Target Display Mode only offered mirrored desktop.

Apple began the deprecation of Target Display Mode in Late 2014, and threw the last shovel of dirt in its face in early 2020. It's dead tech.

Hi @patrycja49, you may find this article helpful: Use your iMac as a display with target display mode - Apple Support

Macs do not have a video in port.