Various links:

Adafruit PiUART
https://learn.adafruit.com/adafruit-piuart-usb-console-and-power-add-on-for-raspberry-pi

Console Cable Description
https://learn.adafruit.com/raspberry-pi-zero-creation?view=all

Linphone with Raspicam
https://www.raspberrypi.org/forums/viewtopic.php?t=107963

Linohone & Raspberry Pi
https://wiki.linphone.org/xwiki/wiki/public/view/Linphone/Linphone%20and%20Raspberry%20Pi/

Video Calling on Raspberry Pi 3
https://www.instructables.com/id/Video-Calling-on-Raspberry-Pi-3/

Video Chat App for RPI
https://raspberrypi.stackexchange.com/questions/85092/video-chat-app-for-raspberry-pi

Video conferencing with RPI
https://www.element14.com/community/community/design-challenges/sci-fi-your-pi/blog/2015/04/27/video-conferencing-with-the-raspberry-pi

Raspberry Pi 3 for Video Conferencing
https://www.reddit.com/r/raspberry_pi/comments/4bx52z/raspberry_pi_3_for_video_conferencing/

Video calling on Raspberry Pi
https://www.reddit.com/r/projecttox/comments/3ncyns/video_calling_on_raspberry_pi/

RPi VideoConference Demo OS
https://www.linux-projects.org/rpi-videoconference-demo-os/

Jitsi Meet
https://meet.jit.si/

Jangouts
http://www.raspberry-pi-geek.com/Archive/2017/22/Open-and-secure-videoconferencing-with-Jangouts

H264 codec for Linphone on Ubuntu
https://www.jaredlog.com/?p=1250

VideoCore details
https://en.wikipedia.org/wiki/VideoCore

Configuring uv4L for webRTC using USB camera on RPI2 raspbian
https://raspberrypi.stackexchange.com/questions/39690/configuring-uv4l-for-webrtc-using-usb-camera-on-rpi2-raspbian

Turn that Pi Zero into a Streaming Camera, Step-by-Step
https://hackaday.com/2017/03/26/turn-that-pi-zero-into-a-streaming-camera-step-by-step/

uv4l-server
https://www.linux-projects.org/documentation/uv4l-server/

Example of custom web app in HTML5 and plain JavaScript: face detection on a video stream from a camera in the network
https://www.linux-projects.org/uv4l/tutorials/custom-webapp-with-face-detection/

How to install or upgrade UV4L on Raspbian Wheezy, Raspbian Jessie & Raspbian Stretch for Raspberry Pi
http://www.linux-projects.org/uv4l/installation/

How to create a SD card for your Raspberry Pi using command line on GNU / Linux
https://howtoraspberrypi.com/create-sd-card-raspberry-pi-command-line-linux/

Setup SD Card for RPI using linux
https://www.dexterindustries.com/howto/setup-your-sd-card-for-the-raspberry-pi-using-linux/

How to install Raspbian OS on SD card in Ubuntu
https://itsfoss.com/tutorial-how-to-install-raspberry-pi-os-raspbian-wheezy/

Setting WiFi up via command line
https://www.raspberrypi.org/documentation/configuration/wireless/wireless-cli.md

Easy GUI Install & Re-Partition Raspberry Pi on Ubuntu
http://mitchtech.net/easy-gui-install-re-partition-raspberry-pi-on-ubuntu/

RPi Easy SD Card Setup
https://elinux.org/RPi_Easy_SD_Card_Setup

Raspbian
https://www.raspberrypi.org/downloads/raspbian/

the Pi Zero W and PaPiRus ePaper display
https://blog.virtualwolf.org/more-raspberry-pi-adventures-the-pi-zero-w-and-papirus-epaper-display/

Raspberry Pi Zero W and Funtoo
https://mtekk.us/archives/guides/raspberry-pi-zero-w-and-funtoo/

WebRTC on Raspberry Pi without Browser
https://stackoverflow.com/questions/32585903/webrtc-on-raspberry-pi-without-browser