Hi @linub,
I do know anything about installing flatpaks but you have another solutions.
The zoom package is in the AUR repository and you can also add it as a browser add-on on your browser.
You can also download the package from the zoom download webpage and run the following command with pacman, which I found in the man pages.
pacman -U /home/user/zoom_x86_64.pkg.tar.gz Install zoom package from a local file.
Otherwise, you can read the following post in order to install tha package from AUR.
‘Arch User Repository - Manjaro’
‘HowTo: Install git Packages Specifically From AUR on Manjaro’
Hope this help, regards.