It's a regular job. I follow a youtuber who was, before his youtube career, a voice actor among other things, and it's a job like any other. Large companies have large advertising budgets, and they usually pay better than, say, film and animation. Voice actors are often signed to agencies, they have profiles and portfolio. Voice is a product and a service, and agencies can be requested to provide samples for a request like a warm, friendly voice for a 15-minute training video. They have contracts that specify where and how the voice will be used, and if the payment is fixed, dynamic based on a business metric or success of the use, or a mix of these, and how much of that does the agency get.
What's interesting is that voices can also be exclusively licensed. You for example Mercedes can request a characteristic voice, and they pay extra so that the actor signs that they will not take another transportation related job, or even any other job for a period. This is how McDonalds has a specific, associated voiceover for every commercial over a period, for example. It becomes part of branding, like a color, a font, or a logo design.