What's the difference between the snap folder in /home/UserName and the snap folder in root (/)?

The /snap-directory contains a file named README:

This directory presents installed snap packages.

It has the following structure:

/snap/bin                   - Symlinks to snap applications.
/snap/<snapname>/<revision> - Mountpoint for snap content.
/snap/<snapname>/current    - Symlink to current revision, if enabled.

DISK SPACE USAGE

The disk space consumed by the content under this directory is
minimal as the real snap content never leaves the .snap file.
Snaps are *mounted* rather than unpacked.

For further details please visit
https://forum.snapcraft.io/t/the-snap-directory/2817

To sum it up, this directory contains the mount-points for your snaps and several symlinks which are needed by snapd.

The directory /home/<username>/snap holds the configuration files for your snaps.