All the policies and procedures are there to, ultimately, protect the company from wrongful termination or discrimination lawsuits. There's not really a govt body that mandates that.

(Handwave around: there are some protected activities that the govt cares if you're fired for, and if you can prove you were fired as retaliation then the company is in trouble, but mostly your manager can just fire you)

In practice, however, most organizations are extremely risk-averse when it comes to wrongful termination lawsuits. And as such, they'll have a lengthy "performance-improvement plan" (PIP) that can last three to six months and is meant to serve as evidence that an employee's poor performance was the reason for dismissal.

There are also, of course, extreme cases where someone can be fired "for cause" when they've committed some kind of egregious violation (e.g., embezzling funds, fraud, corporate espionage).

[1.] https://www.usa.gov/termination-for-employers

Many large US companies also use such procedures. Instead of firing you on the spot, your manager puts you through a three-month "performance improvement program", aka PIP, during which the documentation of your inadequate performance and failure to improve is created, and then you are fired.

At smaller companies in most of the US, a manager can just say bye and you're gone.

Note well: I am not a labor-law lawyer. There may be some nuances and details here that escape me.