I'm not 100% sure of all points, but udevil knows they are USB drives and this inherently means they are removable block storage devices unlike e.g. SATA drives which are treated as non-removable, and it mounts them to a known "auto-mount removable block storage" location. Those differences are all documented/registered in the udev database which is queried by apps; and those apps may choose to show different icons or handle them differently based on different properties, e.g. bus type, mount location. I don't use UPnP so can't say what Kodi does with removable vs. internal drives, but there's probably info in the Kodi wiki or you might need to ask Q's in the Kodi forum.
To prevent auto-mounting you'll need to clone the default rules file to /storage/.config/udev.rules.d/95-udevil-mount.rules and then modify it to create exceptions for specific drives using UUIDs/LABELS or such. You can then create alternative mounts e.g. using a systemd service that runs ExecStart commands to mount things as you need them.