Google has finally added a document scanner to the cloud storage client for iOS and iPadOS - simultaneously with the redesign of the feature on Android.
To start the scanner, open the "Files" section and click on the camera icon — it is located above the "+" icon. To use the function, you will need to give access to the camera. Document border recognition, cropping and alignment are performed automatically, but you can correct something manually. The finished file can be saved as a PDF.
The new feature is already being distributed to users of the "Disk" with a Workspace subscription and standard accounts. If you haven't got it yet, you'll have to wait a bit: usually such updates are distributed gradually over several weeks.
If you have an iPhone or iPad, but you do not use Google Drive, then you can scan documents in the built-in application "Notes".