There is no built-in LiDAR scanner on the iPhone 12, however there are many third-party apps that you can use to add this functionality. I would recommend using the app Lidar Lite 3D Scanner, which is available for free on the App Store. This app uses the iPhone's camera and LiDAR sensor to create 3D scans of objects, which you can then share with others or export as STL files for 3D printing.
To use the app, simply point your iPhone's camera at the object you want to scan, and the app will automatically generate a 3D model. You can then use the app's controls to adjust the angle and distance of the scan, as well as the resolution. Once you're happy with the scan, you can export it as an STL file or share it with others.
I hope this helps!