Cancelling a trip you booked
Cancel from the trip itself; what comes back to you follows the cancellation terms shown on that booking, which are the only terms that govern it.
You can cancel a trip you booked. What comes back to you depends on the cancellation terms attached to that particular booking.
Where the terms live
Every listing carries a cancellation policy, and the booking shows which one applies and what it refunds before you confirm. Those are the terms that govern your cancellation. Not a general rule, and not what a different listing said.
The three policies
Hosts choose one of three: flexible, moderate or strict. They differ in how much is refunded and how close to the start of the trip a refund still applies. The booking sets out the tiers for the policy on it, and reading them there beats remembering them from here.
How to cancel
Cancel from the trip itself. Do not ask the host to cancel on your behalf: a cancellation entered on the host's side is recorded as a host cancellation, which is a different event with different consequences for them.
The deposit and the fee
A deposit is a hold rather than a charge, so it is not money that has to come back to you. The standard service fee is 25%, but promotional and partner terms exist, and the booking shows what actually applied to yours.
The service fee comes back only on a full-refund cancellation — the top tier of the policy on that listing, which is 24 hours ahead on a flexible one, 72 hours on a moderate one and a week on a strict one. Cancel later than that and the fee is kept. Any deposit hold is released as soon as you cancel, not on the date the trip would have ended.