How to Connect MacBook Pro 2018 15 inch to IMAC 24 inch 2021 as external screen?

The 2021 24-in iMac with Apple-silicon M1 is a more-capable computer than your 2018 MacBook Pro.

Do not try to turn your M1 iMac into a display. Connect a TV set or external display to either computer instead.

Hi,

iMac 2021 doesn't support Target Display Mode.

No. Get an external monitor.