Use Imac as Display/monitor

Sorry, not possible in the normal sense...

iMac models introduced in 2011, 2012, 2013, and mid 2014

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Thunderbolt  or Thunderbolt 2  cable.

24-inch and 27-inch iMac models introduced in 2009 and 2010

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Mini DisplayPort  cable.

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

I haven't tried it, but Turn any Mac or iPad into a second display...

https://lunadisplay.com/

https://www.duetdisplay.com/

Duet - Remote Desktop, Second Display, Drawing Tablet, and More (duetdisplay.com)

Review by NAK005, you may not be happy with it…

Secondary Screen Solution - Apple Community

FYI, You can AirPlay your MacBook Pro to your iMac.

Set up your Mac to be an AirPlay Receiver - Apple Support