One might say that the West actually consisted of three frontiers: the miners’, the cowboys’, and the farmers’. Compare and contrast these frontiers.

The statement to which this question refers emphasizes the economic diversity of the American West. One might add many other economic pursuits as well—logging, railroads, and trapping in particular. One point of comparison between mining and ranching is that, while they are popularly remembered in terms of rugged individuals like the "49er" and the "cowboy," they were in fact dominated by large corporations. Cowboys and miners were mostly wage laborers. Farmers, too, experienced the effects of economic consolidation, though they mostly owned their own lands. They quickly found themselves in debt to banks and struggling under the weight of national monetary policy and extortionate railroad shipping rates.
Indeed, the "cowboy" and the "miner" frontiers were also dependent on the railroads, which were the key to American expansion. Another commonality between these economic "frontiers" is that they were a product of immigrant labor, including Irish, Chinese, and Mexican laborers. In short, it makes historical sense to think of the West not as a monolith, but as a collection of diverse economic pursuits and experiences.
https://www.historians.org/publications-and-directories/perspectives-on-history/september-1996/the-west-as-america-a-review-of-the-latest-ken-burns-documentary