Answer:
by supply and demand
Step-by-step explanation:
If a type of worker is in rare demand (usually because of skills required), they will command a higher salary. If a company doesn't offer a salary high enough, it will not fill the post, or at least it will not fill it with competent staff, as skilled workers will obviously go where they can trade their skills for most money.
If a type of work doesn't require any special skill, then anyone can do the job... so there are plenty of workers... and companies can offer a low salary and still fill their positions because there will always be more people ready to do the work.