Screen only mirrors, not extends

please read what he asked

You are presumably attempting to use the iPadOS Stage Manager feature.

Your monitor’s screen resolution will impact your iPad’s available settings and external screen capabilities. 

While undocumented, testing has demonstrated that 4:3 monitors with a display resolution of 1280x1024 and below are currently only usable with iPad screen mirroring; as such, associated settings to disable screen mirroring in favour of Stage Manager are not available in iPad Display settings.

When used with a 16:9 widescreen monitor, with a native display resolution of 1920x1080 or greater, the full set of Display settings - and access to Stage Manager - should become available:

Settings > Display & Brightness > [Displays] Arrangement

If present, tap to expose the related settings:

To enable Stage Manager for the external display, ensure that Mirror Display is disabled.

The iPad User Guide lists the upper resolution limit as 6K - but the Guide offers no lower limit, or suggestion of aspect-ratio, being a factor for Stage Manager compatibility:

Connect iPad to a display with a cable - Apple Support

This discussion thread may be of additional interest:

iPad Extended Desktop not working - Apple Community

In conclusion, your monitor must be capable of 16:9 widescreen 1920x1080 (FHD) to support iPadOS16 Stage Manager extended desktop. Unless the monitor meets the minimum required specifications, you will not see the full range of controls in iPad settings. 

1. Compatibility: Ensure that your iPad and Mac support the Sidecar feature. Sidecar is available on Macs running macOS Catalina (10.15) or later and iPads running iPadOS 13 or later.
2. Connection: Make sure your iPad and Mac are connected to the same Wi-Fi network and have Bluetooth enabled.
3. Sidecar Setup:
4. On your Mac:
   1. Click on the Apple menu and select "System Preferences."
   2. Choose "Displays."
   3. Go to the "Arrangement" tab.
   4. Look for the "Display" section and check the box next to your iPad under "Built-in Retina Display."
5. On your iPad:
   1. Open the "Settings" app.
   2. Scroll down and tap on "Sidecar."
   3. You should see your Mac listed under "Devices." Tap on it to connect.
6. Extended Display: Once connected, your iPad should function as an extended display for your Mac. You can move windows and apps to the iPad screen to extend your workspace.

You are very welcome. I’m delighted that the information was helpful to you.

There were apparent functional changes to Stage Manager sometime around the release of iPadOS 16.4 - although for the vast majority of users, with modern (higher resolution) monitors, the changes were unnoticed. Whether or not these were were intentional or a functional imitation - or potentially the victim of a bug fix - is unknown.

To be entirely objective, Stage Manager’s windowing environment is unusable at screen resolutions of 1280x1024 and below - these resolutions only being found with older 4:3 aspect ratio monitors. While entirely speculative, it is quite possible that Stage Manager has been updated to only permit mirroring with low resolution monitors.

With every iOS/iPadOS release the system evolves.