It works the same as maintaining road infra. Car/fuel taxes are not enough to cover it, so ppl taxes are used. There is no sustainable transport in this regard maybe except bike lanes since these require minimal maintenance after built and have long lifespan, but public transport is better (cheaper to maintain) compared to car infra when accounting nr of passengers transported
Buses need much less lanes since in dense areas one bus line can serve thousands of ppl that live close to each other. Operating buses on all roads is not practical, it'll be expensive taxi on steroids, the scope of the bus is to collect ppl from a bigger area and drop them to a place where ppl also continue walking to their destination
