If you do not have a webcam, nor a camera of those that can be used as a webcam thanks to the applications offered by some brands such as Sony or Canon, you can always take advantage of your Android mobile, and if it's an old smartphone that you don't use, much better, because you can use it even in a dedicated way for it.
Thanks to applications like DroidCam, the process can be quite straightforward and the result is that you have a fully functional webcam in the form of almost any Android mobile that has version 5.0 or higher (a very, very wide range).
How to install and use DroidCam
The first thing you need is download the DroidCam client for Windows from the Dev47Apps website, the creators of this tool. Before installing the program, they recommend that you close all your messaging apps such as Zoom, Skype, Microsoft Teams, etc. to avoid problems or reboots.
The second is install the DroidCam app on your Android mobile from the Google PlayStore, it is a free app that is compatible with the vast majority of mobiles, even the oldest ones. It works through a USB cable or simply with WiFi, which is the simplest, fastest and most convenient method of use.
Once both applications are installed, run DroidCam on Windows and open DroidCam on your Android. From the Android application you will see data such as your WiFi IP and the port used by the application. This data must be copied and pasted into the DroidCam client for Windows in the window that appears with these empty fields:
Once you've pasted the "Device IP" in Windows to match the one in the Android app, be sure to check the "Video" and "Audio" boxes and click "Start". You should see the image immediately in the app. You can even instantly switch between your front or rear camera.
This is the method to use over WiFi and it works the best. You can try using a USB cable, but for this you must activate the developer options in Android and activate USB debugging, it is more complicated and slow and tends to fail. You will do better if you only use the cable to maintain the charge of the device and nothing else.
How to use DroidCam with Zoom
If everything has gone well up to here, chances are that Zoom and any other video calling application you use will detect your new "webcam" and use it automatically. If this is not the case, you just have to open the Zoom Settings by clicking on the nut in the upper right corner of the app window.
Then you just have to go to the section Video and in the drop-down menu just below the word "Camera", you must choose DroidCam Source.