ART is not a hotel shuttle, it is run by the county. A day pass is $5 or $12 for 3 days. It usually begins an hour before park opening (including Magic Morning early days). It only services hotels and the park. Each hotel has a route number, and when you exit at
Disneyland, that is the numbered stop you wait at. For example, the route for La Quinta/Extended Stay is #8. Motel 6 is #9, I think.
As a big city girl, I find the system very simple, running about every 20 minutes. Those uncomfortable with public transit may be uncomfortable. The shuttles are clean and modern. They can fill up at closing time, however.
Only a few hotels have their own. I think Hyatt Regency does and possibly Sheraton.