Have you ever thought about using your smartphone screen as a Raspberry Pi 400 monitor? Well, Redditor bazliamin has showcased this to be possible.
For the build, bazliamin used an Android phone (Xiaomi Pocophone F1), a female USB-A to male USB-C adapter, and a $9 HDMI to USB-A video capture card aside from the Raspberry Pi 400 desktop PC/keyboard.
To get started, first install the USB camera app on an Android device. In this case, the USB Camera - Connect EasyCap or USB WebCam application was used, but there are many others available in the Google Play Store. (According to fellow Redditor BenRandomNameHere, the USB Camera Standard is another option.)
After installing the application on your phone, you have to connect the Raspberry Pi to Android via the video capture card and open the USB camera app. Then select a suitable resolution, and you can even make the display into full screen.
While running this setup, the touch on the phone works perfectly fine. The same can be done on a tablet like the Xiaomi Mi Pad 4 and a 10" Kindle Fire HD.
The basic concept is to capture what's in the Raspberry Pi 400 and have the Android phone show the display as if it were a webcam. More details can be found here.