Santiago cultural circuit bus is a service run by transantiago on Sundays and holidays, it's an inexpensive way to explore the city, for the normal bus fare (CLP 700) you get to move around 18 stops in Santiago Centro.
The circuit starts at 10 am and ends at 6.30 pm.
You can pick the bus at Estacion central bus station or contact transantiago and they'll you directions on time and location of pick up.
Bus fare is CLP 700 for adults, kids get free rides. Payment can be either done by BIP card (same BIP card you use for the metro) or cash.
The metro is not part of Santiago cultural circuit and I actually found out metro staff know very little about the service when I tried to seek more info about the circuit from them.