To use modifier keys (e.g. Ctrl, Shift, Alt) on the Touch Keyboard on Windows 10, you need to select the option for its ‘Expanded Standard Keyboard’ layout.
Here below is an article from Microsoft that explains how to use the touch keyboard:
In Notion Mobile, the On-Screen Keyboard (which is different from the Touch Keyboard) works as expected. In Windows 11, both the Touch Keyboard and On-Screen Keyboard work as expected.