2019 16" Macbook Pro as Second Display for 2021 24" iMac

You can't really do what you're describing with only macOS' built-in features, but there are a few solutions that might achieve similar results.

• Universal Control: You can drag your mouse cursor from the edge of your screen on one Mac to another. You can't drag windows from one Mac to another, but you can share PDFs, photos, and other files by dragging them from one Mac to another. Your workaround here would be to sync your tabs/files with iCloud (or Google/Microsoft/etc.) and open your application of choice on your MacBook Pro. To do this, you'll need to make sure both your Macs are running the latest version of macOS Monterey or macOS Ventura, and that both devices are signed into the same Apple ID. Universal Control: Use a single keyboard and mouse between Mac and iPad - Apple Support

• AirPlay: You can set up your MacBook Pro to be an AirPlay receiver, then AirPlay from your iMac to your MacBook Pro. Your MacBook Pro will replace the HDTV in this setup: Use AirPlay to stream what’s on your Mac to an HDTV - Apple Support. To use your MacBook Pro as a second display, select "Use As Separate Display" in Control Center or System Settings. To do this, you'll need to make sure both your Macs are running the latest version of macOS Monterey or macOS Ventura.

• Third-Party Software: You can install third-party applications, like Duet Display, on your Mac to accomplish effectively the same task you're describing. These solutions may or may not support your system configurations and likely will cost extra.