Montreal's metro is really the basic way to get around - there are four lines that cover a good chunk of the city. Buses complement the system well, so by combining the two you can get almost anywhere without any issues. For two weeks, you can buy individual tickets or get a rechargeable card, but honestly there are also short-term passes like 3 days or a week that might be worth it if you're planning to move around a lot. You'll find all that at metro stations.
It's efficient for real, especially if you stay in well-served neighborhoods. Downtown, the Plateau, Old Montreal, all that's easily accessible. The advantage is you can check schedules and routes on their app or right on the website before you leave, so no surprises. The stations are spaced out pretty well and the system isn't complicated to figure out once you get the hang of it.
One thing to know: in the fall in Montreal it's already pretty cold, so having a good transit app on your phone is handy so you don't have to stand outside waiting for the bus longer than necessary. Bring good clothes. And check the schedules for the last departures in the evening depending on the lines - it's not the same everywhere.