I have observed the ferries and TTC monorail start as late as 20 minutes before park opening, and ridden the resort monorail just over an hour before park opening. The resort monorail needs to pick up people going to early character breakfasts in the park.
The delaying of the start of transportation from TTC to MK inhibits day guests from getting into early entry and also allows resort guests to line up at the turnstiles first. Except it makes sense to start the transportation earlier if there are huge crowds waiting at the TTC.
If you are staying at one of the monorail resorts, you would use the resort monorail.
When I used to work in Monorails, they would do the "First Family" ceremony around 8:45... which would start the official first Monorail and Ferryboat. However, the Resort monorail does start service at 7:00am daily.
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