If you don't have a webcam in reach or like to have a higher resolution than your cheap one:
Unfortunately there seems to be no open source solution by now :-(
As Windows or MacOS users you will find as usual a greater variety of applications for this project.
As Linux user I first tried DroidCam, but
you need to compile the linux app and the driver by yourself and if you manage that you will realize that
the standard version is limited to 640x480 resolution.
The "Pro"-Version seems to cost around 5€ to give you max 1028x720.
The full resolution of your cam you will receive with Iriun Webcam out of the box.
There's documented an inApp purchase of a pro version but I didn't find where to buy it or what it would include.
There is driver software for Ubuntu 18.04 or newer, Windows and MacOS.
As soon as you open the app on your device and the driver app on your PC and you are either in the same LAN or plugged the phone via USB into it (USB Debugging provided) you will receive the phone's cam as webcam (Linux: /dev/video[n]).
The quality of the stream is high level (depending on your devices cam of course) compared to average webcams. I realized no relevant lag. It broadcasts realtime with 1024x720.
If s.o. knows about a solution with completely open sources I appreciate some links 👍️