If you are going for your temporary residency visa in Chile, you’d better get used to making a lot of trips carrying papers around the city.
In my opinion the best way to learn a new city is to rent/buy a cheap bicycle and just cycle around the place. The best navigation resource out there for Santiago is mapcity.cl. Google Maps is inaccurate sometimes with Santiago, but MapCity is always spot on.
Taxis are almost all metered so being ripped off by a taxi driver isn’t much of a concern here.
Santiago has an excellent, fast and efficient metro system. You will need a Bip! card to use it, you can purchase one at a kiosk in any metro station. The card itself costs CLP$ 1.500 and you can charge (cargar) it with any amount you like. A trip on the metro usually costs about CLP$ 600 and the card can also be used to pay for bus journeys. Interestingly, the cards also allow you to drop into a negative balance, you can actually use your card for about two bus journeys below zero before it stops working.
The opening hours for the Metro can be found here. It depends on the station, but they are usually around 5:30am to 11:30pm.