How can I use my late 2009 iMac as a display for my MacBook Pro?
See this link to make sure your setup qualifies.
Use your iMac as a display with target display mode - Apple Support
See this link to make sure your setup qualifies.
Use your iMac as a display with target display mode - Apple Support
I saw that so my iMac seems to be okay but my MacBook Pro has macOS Monterrey. Does it still work? What kind of cable do you suggest? Minidisplay Port to hdmi?
No. As the information states, you have to have Catalina or earlier installed. Also a 2019 or earlier Mac.
So I am out of luck
To my knowledge, yes. That's what Apple officially supports. You can always do a web search and see if anyone else managed to make your setup work.
No, you are not you could download duet, an external app that may allow you to achieve this.
Do you have an English site maybe? I'm not all that good at Chinese yet.
At the bottom right, click on the button and choose the word ‘Language’ and it will come up in English.
how can i use my imac as a display
