How was the contrast increased with film? One technique is given in this answer and involves Selenium. Are there other methods that were/are used to increase contrast in a photo taken on film?
Most commonly, you used paper with different contrast grade.
There are fixed grade papers, normally available in grade 0 to grade 5. There are also variable grade papers (aka Multigrade), where you change the contrast with different filters that gave different colors.
Push processing (extending the time the film spends in the developer) increases the contrast of the negative or slide. If you're shooting roll film, you pretty much have to push or pull the whole roll. But for sheet film shooters like Ansel Adams, it was a pretty good way to adjust contrast—terms like "N+1 development" in the Zone System refer to this method of adjusting contrast. (You also have to know, prior to development, whether you want to increase the contrast. That sort of planning was Adams' forte.)
By using filters when exposing on to the photographic paper. With B&W they were this orangy color of different densities. You would stick the in front of the projector's lens and then expose on to the paper.