They tend to mention generic things like "great culture", "career plans", "managers who listen", "transparency".

You are absolutely right, it's all fluffy and intangible. I mean, it's all obviously good, and also every company says this about themselves. Whoever heard of a company advertising "toxic culture", "you'll never get a promotion", and "nobody knows anything about the company's health until it's bankrupt"?

The way it works is a questionnaire is created by the rating agency, meant to cover a wide degree of workplace factors. The do tend to cover the ones that have some academic rigor, like questions about autonomy or career growth, beyond the fluffy ones. Weighting of these factors hasn't ever been transparent to me as an employee. And it does seem to be pay-to-play to be considered, but there is an attempt at objectivity. But for your example, if 90% of the employees say they enjoy coming to work everyday, or they would recommend working to in that workplace to a colleague at a different firm, then independent of the factors enabled such opinions, that company did something right, modulo people just giving bullshit answers.