Regarding tools, Citymapper is often the best if you’re in a city that they cover and you have internet (it works offline during the trip, but not otherwise). They actually send people to scout the walking distance between platforms, exits etc and take that into account when suggesting routes, as well as telling you which part of the train to board in order to be closest to your transfer/exit at the other end. As a result of the work involved in that level of detail, they only cover certain cities. Google Maps (or Apple Maps in China!) sorts out the rest.

For fully-offline I’ve found maps.me to be quite good, again, where it has public transit coverage.