unionfs-fuse is not a real file system like the kinds used to format partitions on hard disks… It is a virtual file system ... i.e. a utility to merge multiple mount points / directories under one mount point / directory … It can be installed from repositories like any other package. Therefore, it is not something to sort out during the installation of Ubuntu ... you should install Ubuntu as usual and once the system is up and running comes the part of installing support for UnionFS and merging directories / mount points with it.
However, unionfs-fuse is not the only or most robust tool of its kind ... aufs is a recommended newer implementation of UnionFS and is fairly simple to install and use.
You can install aufs support like so:
sudo apt install aufs-tools
You can use it like so:
sudo mount -t aufs -o br=/FirstDirectory=rw:/SecondDrectory=rw:/ThirdDirectory=rw none /UnifiedMountPoint/
Where /FirstDirectory, /SecondDrectory and /ThirdDirectory ... etc. are multiple directories / mount points (AKA branches) that needed to be unified under one directory / mount point ... they are separated by a colon : and =rw right after the directories paths is an option to set read and write permissions for those directories.
And where /UnifiedMountPoint/ is the directory / mount point under which all the branch directories are unified and accessed. This Directory /mount point must exist / be created beforehand.
