My guess would be that for a salsa that strives to be pretty much the same, but with different levels of heat, they probably vary the ratio of certain peppers, or they vary the amount of the same peppers.
For example, Pace lists the exact same ingredients for their mild, medium and hot chunky salsas.
What Pace does is vary the proportion of standard hot jalapenos to other jalapenos bred to have no heat. This means that the flavor of the salsa doesn’t vary, only the heat level.
(proof Pace uses heatless jalapenos)
Others might adjust with straight capsaicin, while still others may just vary the proportion of something like bell peppers vs. jalapenos, since they’re similar in flavor.
Still others will come up with different recipes using different peppers (and heat levels) entirely.