Without knowledge of the circuits that are already in the garage, it's pretty hard to say what wiring is appropriate. I'm not understanding the need for "one continuous piece". Are you concerned about the number of junctions? You can either run conduit with individual wires inside (which provides the most flexibility) or a cable designed for direct burial.

The only advantage of "one continuous piece" would be appearance, not usually important for buried conduit.

Waterproof wiring is required since ALL buried locations are considered wet, and ANY buried conduit eventually fills with water.

There are specific rules about how many circuits can be run between separate structures before a panel is required.

ONE circuit is a common limit between detached buildings.