you don't have too, it's all pretty informal besides taxes and I'd verification. no contract says you have to do X or can't do X, it's more rules or policies, which are often very bendable. you either take it or leave it, they can fire you as they see fit. just as youre not required to give 2 weeks notice. so if you never ask for raise or promotion, good chance you may not get either.

You get an offer letter that spells some of that out, but it isn't a binding contract.

An employment relationship in the United States is presumed to be “at-will,” i.e., terminable by either party, with or without cause or notice. Indeed, a majority of employees in the United States are employed on an “at-will” basis, without a written employment contract, and only with a written offer of employment that outlines the basic terms and conditions of their employment.

Source

Title, start date and time, job title is about all that's on the "contract".

Rolls and responsabilities, not anything else, are not defined.