Where do backpackers leave their stuff while exploring a city?
Here I know you go to the other side from load, depart to the cargo area of most buss stations. There you can rent a secure space to leave things for a fee. Also just outside of most airports here there is storage you can rent. Just follow the sign that say's check all weapons before entering airport property. Or rent a room. Leave it there.
Many major railway stations have storage lockers.
Most youth hostels have lockers. Hotels most certainly will provide a place for luggage.
Some places, at least used to, have storage lockers, like train stations
On their back?
In their secure hotels.