While being in a video conference, and you are the one who is doing a screen recording, you want to limit what is in the video to only a set of open windows.
In Linux you can create as many layers of your desktop as you want. Install Xephyr. It actually is already in the ubuntu package manager available.
sudo apt-get install xserver-xephyr
Create a new layer with the ID 1. Your running desktop got already ID 0, so the next number is 1.
Xephyr :1 -ac -br -screen 800x600 -reset -terminate
Add a window of an application to the layer.
A very lightweight, and also easy to use window manager.
sudo apt install icewm
Record your screen. For details of how to do this in Linux, move on to the related post here on my site: Screen Recording