I have never heard anything from any Valve interview to the effect of how specific pay rates for specific employees are actually set.

I think (as in, this is my semi-informed speculation) it is closer to:

You get hired to fill a specific role, with a specific pay rate (Valve still like, lists specific job openings with specific role requirements), and then uh, you have tasks within that realm that are fairly clearly your responsibility... but at the same time, everything going on at the company is essentially one big github issues list, a whole bunch of feature requests and bug reports, and everyone can contribute to any of those, assuming they've got their core responsibilities covered as well.

(This is appealing to anyone who has ever worked in a tech corp or really any corp ever, because the standard way this works, at least in American corps... is you get hired for a role, that often has an exceptionally all encompassing job desc, or an absurd number of requirements... and then your boss and their boss just continuously, hierarchically, assign you new and more ever growing responsibilities...ie, your job has 'scope creep' the way a poorly developed game does. Valve inverts this and makes the 'scope creep' much, much more voluntary for the employee.)

(If you've never worked a software dev focused or adjacent role, you would be amazed at how often and regularly management and C Suite just demand you learn and do things entirely not in your job description, just treat you as a 'computer person', as if a heart surgeon is also a brain surgeon and also a metabolic expert and also a pharmaceutical designer.)

Like, there are probably still yearly reviews / the ability to try to negotiate yourself a raise... but I don't think there is some kind of... formal, everyone votes in some sense on how much everyone else should be paid.

Valve is still a private company, not an actual worker cooperative that votes up or down an entire business budget for the whole year or w/e.

They are just a lot more flat of an organizational hierarchy, have a lot more unconventional work culture, but still ultimately a private corp.