We have installed proxmox but didn't partition the disks beforehand. Now we would like to make /var/ be on its own partition. I don't even know where to start.
The Proxmox VE installer chooses a reasonable partitioning scheme itself and does not let you deviate from it much. If the existing settings are too restricting for you, you will have to install Proxmox VE on top of Debian Buster.