These three surfaces do not hold anything of their own. They are views over the cards already on your boards, arranged by when they are due.
That is the point. A separate to-do list that lives beside your boards is a second place to keep in sync, and you will lose.
Planner
Seven days across, with an unscheduled column beside them. Drag a card onto a day to set its due date; drag it between days to reschedule; drag it back to unscheduled to clear the date.
The unscheduled column is where the week actually gets planned. Everything without a date sits there, visible, until you either give it a day or admit it is not happening this week.
Today
Overdue first, then due today, then what is coming. Cross-board, so a personal errand and a work task sit in the same list, which is the honest version of a day.
Press G then T from anywhere to get here.
Calendar
A month grid for the shape of a period rather than the detail of a day. Useful for spotting the week you have quietly overloaded.
Press G then C.
Checklists
Standalone checklists are the other thing in this family: a shopping list, a packing list, a pre-flight for something you do repeatedly. They live outside boards because not everything is a project, and they can be saved as templates and re-run.
Press G then L.
Google Calendar
Due-dated cards can push to Google Calendar, and events can pull back as cards, with recurrence rules preserved in both directions. Neither side is the master. See Integrations.