I put together a quick script that counts how many entities of each kind are in the field.
You only get trip updates from your operator, there are no vehicle positions or service alerts in the feed you provided. Trip updates tell you if the trip is running late or not or give you the updated departure time from the next stop based on the current position. So in this particular case they use the GPS positions to estimate these details but do not actually expose the details to you.
FeedHeader {
gtfsRealtimeVersion: '1.0',
incrementality: 0,
timestamp: Long { low: 1656685866, high: 0, unsigned: true }
}
Entities: 433
of which Trip Updates: 433, Service Alerts: 0, Vehicle Positions: 0
You'll have to either find another API source for the GPS positions, reverse engineer one of their apps if there's any that show the positions or estimate the position based on schedule adherence and the shapes defined in the static GTFS for the trips.