The following assumes you have a debian template.
VSCode:
you can either download the .deb in a dispvm and then transfer it to the template or you can install the snap backend in the template and then download the snap package in the app vm.
Alternatively VScodium provides a repo (scroll down to Debian / Ubuntu (deb package):
you also have a couple of options to accomplish that: you can either use the dispvm approach or you can use qrexec to download it directly from the template:
curl --proxy http://127.0.0.1:8082/ https://gitlab.com/paulcarroty/vscodium-deb-rpm-repo/raw/master/pub.gpg | sudo tee /usr/share/keyrings/vscodium-archive-keyring.gpg
then add the repo:
echo 'deb [ signed-by=/usr/share/keyrings/vscodium-archive-keyring.gpg ] https://download.vscodium.com/debs vscodium main' | sudo tee /etc/apt/sources.list.d/vscodium.list
and install:
sudo apt update && sudo apt install codium
Unity:
Just checked the Unity docs and they also made available a repo, so you can follow the steps above. You can either specify a different keyring name (ie unity.gpg) or remove that part and use apt-key add - instead and remove the [ signed-by= ] part from the repo.
More info: