How do I stop syncing app downloads between iPhone and iPad?

This is easily achieved by modifying your device settings.

Assuming that you have updated your iPhone and iPad to iOS/iPadOS 18.x:

Settings > Apps > App Store > [Automatic Downloads] App Downloads - set to OFF

For iOS/iPadOS 17 and earlier, the settings path is slightly different:

Settings > App Store > [Automatic Downloads] App Downloads - set to OFF

they don't really sync as if you delete an app on one device it's not deleted on the other

only thing it does it default download apps bought of downloaded with the same appleID on other devices

you can disable that

How to manually update apps from the App Store - Apple Support