Fuel, mileage and bringing the car back
Mileage allowance, fuel expectations and house rules are set by the host on the listing, and it is that listing your trip is measured against.
Almost none of this is a platform-wide rule. Each host sets it on their listing, and the listing you booked is the one that counts.
Mileage
A listing states a mileage allowance and what is charged beyond it, or states that mileage is unlimited. Read which before you book rather than on the day you hand the car back.
Fuel and charging
The listing says how the car should come back. The common arrangement is to return it as full as you received it. For an electric car, look for a percentage instead, and for whether a cable is included.
Smoking and cleaning
Smoking is a house rule set per listing. Where a listing does not allow it and the car comes back smoked in, the cleaning can be settled from the deposit against the host's evidence from the handover check.
Photograph the car at both ends
Take your own photos at pickup and again at return: the odometer, the fuel gauge or state of charge, and the car from each corner. Your photographs are what you have if a charge is raised afterwards.
Anything settled from the deposit comes with a written reason you can read, and support decides it rather than the host.