I did my PhD in physics in the USA, and for all of my fellow physics grad students, we not only had free tuition but we were given jobs (they didn't pay much but it was enough to live on). Ie, students were either given Research Assistant roles from their Adviser's funding, or Teaching Assistant roles by the department. The lucky ttudents with fellowships didn't need to teach at all and could focus full time from day one on research. (Of course this gives those fellowship students more ability to write papers early in their career, boosting their ability to win more fellowships later on, creating academias's own 1%/99% inequality, but that's a separate issue).

This was physics, and my understanding is that this is the case for most of the sciences.