Drivers

Ideally, your webcam will be supported by the UVC (USB Video Class) drivers that are included in recent kernels. You just need to build the correct drivers as mentioned here.

To test your webcam, you can use the luvcview application (in Portage), although you may have to specify a different format if you get an error:

luvcview -f yuv

To use the microphone of your webcam, you will need USB Audio support (Device Drivers -> Sound card support -> USB Sound Devices -> USB Audio/MIDI driver). Then it should work out of the box with PulseAudio.

IPU6 / Intel integrated webcams

Those webcams seem to use directly the Intel processor (Alder Lake for instance). Thus they don't appear in lsusb or lspci. If the webcam works on Windows, it's best to use the Device Manager to get details about the hardware device.
As of August 2023, Linux support for these webcams is very limited. Drivers are not upstreamed to mainline kernels; another issue is apparently that those drivers don't use the standard Linux V4L API.
It seems such a webcam is present in my Lenovo Yoga 9i Slim laptop model. I could not get it to work yet.

Sometimes a hardware switch will disable the camera. For instance, the Lenovo Yoga 9i Slim has it, on the right side of the laptop.

In Ekiga, in order to use v4l2 the pwlib / ptlib (ptlib is for recent Ekiga versions) dependency must be built with the v4l2 USE flag on.
I did not manage to get full screen working in Ekiga 3.2.5 yet. The key is F11 but the option is greyed out in the menu.