I have tried the answer of this post:
sudo snap connect telegram-desktop:camera
However, I cannot make my external camera the one which telegram uses in the video calls. I cannot see the option to select the input devices neither for the micro nor for the video.
By default, Telegram only uses the in-build camera of my laptop, but I want to use my external camera instead.
I have the last snap version of telegram and Ubuntu 20.10
I have found the video call settings. It was a matter of selecting the default input video there. However, I think video call setting is a little hide in the settings menu.
Settings → Advanced → Call settings (in system integration section)
So, with a more recent version of Telegram Desktop (3.3) it's like this:
Calls
Call settings
and from there you'll manage ;)
In my case I had to go to Settings → Calls → Settings and I could select my external webcam there.