connecting Macbook pro to a Imac and use this as a monitor.

The 2020 MacBook Pro does not support Target Display Mode.

iMac is limited to only certain models from mid 2014 or older, and are said to require the iMac to be running High Sierra at newest.

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

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iljab wrote:

In the new IOS there is no arrangement tab to see the exrternal monitor

The macOS has never supported using the Mac as an external monitor.

It is fairly new to allow a qualifying iPad to share a Mac screen:

Use your iPad as a second display for your Mac with Sidecar ...

iljab wrote:

I have a Macbook Pro and an older IMac. I ish to use the latter as an external screen for my macbook.

̶I̶t̶ ̶i̶s̶ ̶s̶a̶i̶d̶ ̶t̶h̶a̶t̶ ̶t̶h̶i̶s̶ ̶s̶h̶o̶u̶l̶d̶ ̶w̶o̶r̶k̶.̶ ̶(̶I̶ ̶r̶e̶c̶e̶i̶v̶e̶d̶ ̶t̶h̶a̶t̶ ̶a̶s̶ ̶c̶o̶n̶f̶i̶r̶m̶e̶d̶ ̶f̶r̶o̶m̶ ̶m̶y̶ ̶a̶p̶p̶l̶e̶ ̶s̶t̶o̶r̶e̶)̶.̶

The macOS has never supported using the Mac as an external monitor.