A "moped" is a motorized bike with pedals - it's a drive style, not a body or frame configuration.Scooter as in moped-style? Sit down feet together in front?
"Sit down feet together in front" is a description of a configuration with a step-through frame, popular with the type of motorcycle (not moped) which is commonly called a "scooter"... like the classic Vespas. "Scooter" itself means so many different things to different people that it effectively means nothing at all.
"Underbone" is none of the above, but is a construction style which also implies other features, with different implications to different people.
None of these terms contribute much to useful communication, since they are so ambiguous.