> But you don't really do 'classes' as a PhD student.
In the US almost all programmes begin with a two year long Ph.D. student period at the end of which, assuming you pass your qualifying exams you can drop out with a Master’s. Once those are done you absolutely must focus on research though if you haven’t started on it already you’re behind. After the Master’s you’re a Ph.D. candidate. Somewhere in this period, if you’re doing it right you’re very likely to become the world’s leading expert on your particular, extremely narrow field. Then, or while becoming that expert you write up your thesis, defend it and get the degree.
In the UK it’s still possible to go directly from Bachelor’s to Ph.D. candidate and be done in three years but the trend is away from this, at least in the exact and social sciences.
In my field, it seems most PhD students spend the majority of at least their first year taking classes. But it's true that the overall course load is significantly less than bachelor's degree. Of course you don't come into a PhD program already being an expert. Even if you are, there's certainly much to learn about related fields.
But you don't really do 'classes' as a PhD student. At that point you should be the world expert in your subject so who would be teaching you :)