how can I use my iMac as external monitor with MacBook pro

Not possible with a MacBook Pro running Big Sur.

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

Will it be possible in the future? It is supported on older versions.

Is there another way to use the iMac and MacBook Pro together?

Actually if you can run Monterey you can Airplay to Macs.

Do a Google search for "airplay to mac monterey".

Thank you for the quick response. I will definitely do that.

You're welcome.