snaps are compressed (readonly) squashfs filesystem images. when you build a snap your files get copied into that filesystem …
/snap/foo/current/ is the place where the squashfs file of the “foo” snap gets mounted (this corresponds to $SNAP) at install time …
at runtime /var/snap/foo/common and /var/snap/foo/current can be used as writable space (these translate to $SNAP_COMMON and $SNAP_DATA) …
the dump plugin just defines where in your squashfs your files will go, nothing else … if you want a file somewhere in the writable space at runtime, you can copy that file from the squashfs to the writable place from either an install hook or from a command-chain wrapper that runs at startup of your app.