iMac screen sharing or using as a monitor

Some iMacs support Target Display Mode. Target Display Mode lets you use the built-in display on a supported iMac as an external display for another Mac.

Apple has published a support article on how to do to this.

https://support.apple.com/en-us/HT204592

You'll see a chart with the supported devices, like this.

The caveat is that iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays. So it looks like your 2019 will require you to use Screen Sharing to accomplish this.

No iMac since 2015 will support Target Display mode.

Enabling Screen Sharing:

https://support.apple.com/guide/mac-help/turn-screen-sharing-on-or-off-mh11848/mac

No, as previously mentioned. Please read Use your iMac as a display with Target Display Mode and you will see your iMac will not do this.