I stand to be corrected though but I don’t think I can studio is available for Ubuntu. However, I would advise you to use DroidCam. It offers a range of video quality that are fairly decent and it is easily customizable.
To set up, you just need to install DroidCam on your phone and DroidCam client on your Ubuntu. Set up a WiFi network and connect both devices. Go to OBS Studio and select DroidCam source in the option to add a Video Capture Source.