Drivers are usually paid their "base" salary -- or you could say the biggest chunk of their earnings -- in one of the following ways:

Mileage (what we'll concentrate on here)

A company pays a certain amount per mile traveled, with exceptions.... The majority of company, over the road drivers are paid this way.

A Percentage of what the load pays.  This type is more common with flatbed and with Owner Operators than with company drivers.

Salaried / Hourly drivers are typically local drivers and/or those who run a specific, set route every day and/or stay within a certain territory.

By the Load  This type of pay is seen often with gasoline/diesel haulers.