Can't see my iMac 2019 as an external monitor with default resolution which is 3200x1800

You won't see the iMac as an external monitor.

It cannot be used as such. It's a computer, not a monitor.

Target display mode is a feature that was available only with certain older iMac models.

The feature has been deprecated by Apple and is no longer supported in hardware or software.

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

Native support in the macOS for the use of an iMac as a directly connected monitor for any of the newest Macs is not provided by Apple.

Are you using AirPlay? If so, I believe the highest resolution for that will be 1080p with a 16:9 aspect. It doesn't support the max resolution of the iMac.

Please see: Stream audio and video from your Mac with AirPlay - Apple Support

I am aware of third-party solutions such as Luna Display or Duet Display, but support for those products will come from their respective manufacturers.

Jufs wrote:

I'm able to use, it's just a resolution issue.

What connection and process are you using?

Screen Sharing is limited by the slave computers resolution.

Third Party software like Luna Display and Duet Display have inherent resolution limitations.

As mentioned, those Mac’s do not support Target Display Mode.

Sorry, it is no longer possible to use Macs as a monitor for the newer Macs. You will have to purchase a monitor.