PhD programs are very different. The environment Karpathy describes is fairly similar to what I saw as a math PhD in a good school, but not an ivy. My theoretical physics PhD friends had the same setup as I had, but experimentals lived in a different world, long hours in the lab every day, including weekends.

My advisor was well established, tenured prof with a number of students. I had to teach, but the effort was light. We taught large, basic courses that are boring for tenured profs. We usually requested the same 1-2 classes to teach and after the first round had all the materials (homework, quizzes, etc.) and could teach on autopilot. University gave us undergrad graders to grade assignments but I never used them since I wanted to see what my students wrote. Which is a testament that the load was light; if I was drowning I would use all free help I could.

But there was a cult of academia. "Get an academic job or you are a loser" mentality was prevalent. My advisor was disappointed, but OK when I decided to go into industry after PhD, but a friend's (Physics PhD from Harvard, CEO of a profitable startup now) advisor does not talk to him anymore because he did not stay in academia.

And I only realized long after finishing my PhD how incredibly lonely PhD path is. You live in your bubble for years, with minimal interactions outside a few other folks at the same grad school. Stipend was enough for basic living, but not much else. No good vacations, ski trips with friends, etc. And a few somewhat creepy characters that grow in this lifestyle. This is all surmountable, but the mental toughness required is certainly something to keep in mind. I did not have that mental toughness, but was an introvert, which helped a lot. But looking back I see that I also could have gone off the rails. My 2c.