Linux Filesystems – Which Support Reflinks

resize2fs is able to both grow and shrink ext2/ext3/ext4 filesystems.

The word "filesystem" is somewhat overloaded, and I think that might be confusing you. In one sense a "filesystem" is the format in which files are written to some medium (e.g., a partition on a disk). In another sense, a "filesystem" (or more specifically, a "Virtual Filesystem") is an abstraction provided by the OS that presents a set of files (regular files, directories, etc). An OS can read the on-disk filesystem and present a filesystem abstraction.

The files presented in the filesystem abstraction can be stored on disk (e.g., ext4), on some other host across the network (e.g., cifs, nfs), or elsewhere. Something like mergerfs takes multiple sources of files and presents them as if it was a single source. From their website "mergerfs logically merges multiple paths together. Think a union of sets."

Take a look at the mergerfs website, they have a nice description of what that does.