Learn Spanish (Spain): Trabajo de nueve a dos.

You can absolutely say desde las nueve hasta las dos, but using de followed by a is a much shorter, more natural way to talk about the start and end times of a regular daily schedule.

Why aren't there any articles before the numbers, like de las nueve?

When we use the de... a... structure to talk about schedules, we usually drop the article las and just say the numbers. If you choose to use desde and hasta instead, you must include las.

Does dos refer to 2:00 AM or 2:00 PM here?

It means 2:00 PM. In Spain, a typical morning work or school shift often runs from 9:00 AM to 2:00 PM. The context makes it clear without you needing to add de la tarde.

Could Trabajo mean the noun 'job' in this sentence?

No, here it is the verb meaning 'I work'. While el trabajo does mean 'the job' or 'the work', the lack of an article and the context of a schedule tell us this is the 'I' form of the verb trabajar.