I’m unsure of your expectations - but consider the following...
Unlike PC and Mac computers, with which you are doubtless familiar, iPad doesn’t have the equivalent of an extended desktop - supporting content display across multiple displays (whether this is an external monitor, TV or other computer).
iPad has Screen Mirroring - that reproduces what you can see on the iPad display - on an external TV or monitor. Screen Mirroring does exactly as its name implies; the iPad screen is reproduced on the external display, preserving both the content and aspect ratio of the iPad screen.
AirPlay extends capabilities of iPad - permitting display of playing media content (either download or streamed) to an external TV or monitor, at full resolution and aspect ratio of the source material, independently of the iPad screen. This capability, however, does not extend to other Apps.
In a touch-interface environment, an extended desktop contextually makes no logical sense; you’d not be able to interact with Apps displayed on an external screen. Screen Mirroring displays the iPad screen on an external display - but all interaction with the App is via the touch screen of the iPad.
Consider that, for the most part, a Mac computer can access the same services and content as your iPad - either directly, or perhaps using iCloud and its associated Continuity and Handoff services. Again, these features make the need for extending a display from iPad to Mac somewhat pointless.
As you are aware, a Mac does have the ability to use Sidecar - this permitting use of iPad as both a second display for Mac and as a graphics tablet. For older models of Mac, that do not support Sidecar, there are third-party alternatives such as Duet Display.
If you still need to display your iPad screen on your Mac, it might be worth taking a look at Reflector3. Perhaps this will be suitable for your need:
https://www.airsquirrels.com/reflector
I hope this information and insight is helpful in addressing your question - if only in promoting some additional thought.