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STEM Ph.D.s are like jobs. You get paid a stipend (the financial package typically requires you to work as a TA/RA).
A few data points. My Ph.D. program paid me $30k/yr tax free over 4 years. Prestige scholarship holders were able to get even more, maybe $45k-50k. (my numbers are old -- I got my Ph.D. nearly a decade ago)
I've had friends at Princeton who received packages in the range of $28k-$35k/yr from their engineering depts.
Not a huge amount of money, but enough for a single grad student to survive on. I have a friend who did a Ph.D. in Math at Stanford, and he didn't have to pay cent.
Debt is generally not a common issue for STEM Ph.Ds.
Humanities/Social Science Ph.Ds. on the other hand are a completely different ball game. I've heard that many live in relative penury and rarely finish on-time (taking anywhere from 5-7 years). Humanities Ph.Ds. please add data points.