“Adequately” is correct. Typically at least for a STEM PhD program the university pays tuition and fees, and pays for health insurance, and provides a stipend. The term “stipend” refers to money that the university pays to the student to cover things like rent and food and incidental expenses.
You might think of a STEM PhD program as a very interesting research job that pays badly, but if your research succeeds then eventually they give you a doctorate (and it does pay something).
The stipend typically is enough to live frugally in the area where the university is.
Some small amount of parent financial help to the student is not typically needed, but can help. Given that a PhD can easily take 6 or 7 years, living poor for 7 years requires quite a bit of dedication on the part of the student. A small additional amount of funds might allow the student to live a bit better (eg, maybe go out to eat occasionally). Also, a 6 or 7 year delay in starting to fund retirement plans such as a Roth IRA can make an unfortunate significant difference 40 years later when the former student is getting old and ready to retire. However, apparently Congress decided a while back to clarify that the stipend counts as income for purposes of funding an IRA (this assumes that we are in the USA). Unfortunately the stipend is not enough to both live on and fund an IRA. A small amount of financial support from the parent to the child could allow the student to fund rent and food and also fund for example a Roth IRA. In the short term this really does not impact the student. 40 years later it is likely to matter. I also think that teaching our children to put money into retirement funds is a good lesson for them to learn.
Thus some small amount of financial help from the parent to the PhD student can be helpful, but note the word “small” and also note that for a PhD student this is really not actually needed. The student typically can live just on the stipend alone.
Master’s degrees are different. They are shorter in length (one or two years), and are typically not funded. I have heard of a few cases where the student gets some financial aid, but this is probably fairly unusual and even if it happens is not enough aid to cover the full cost.
Another issue is that some students do not get accepted directly into a PhD program after getting a bachelor’s degree. It is common for students to work for a year or two or three hopefully at a research job (since research experience can help a lot with PhD admissions). However, some students get a master’s degree after getting their bachelor’s degree before going for a PhD. Master’s degree programs are typically not funded, and can be full pay (eg could in some cases cost close to $100,000 per year for two years – note that a research based master’s degree often takes 2 years).
Some STEM fields (eg, engineering) do not really need any graduate school at all. For engineering and math there are some one year master’s degrees which can be valuable but which are not needed to have a good career. Working in high tech for my entire life I have only met a small handful of people who had a PhD, and mostly they just did the same job that the rest of us did (whether better or worse depended upon the person, but probably not on the degree). I have known a larger number of people, but probably still a minority of coworkers, who had a master’s degree. Personally as a math major I did get a one year master’s degree, and I did find it to be valuable.
For some science careers a PhD can be helpful. For example if you get a bachelor’s degree in biology (or probably I expect in chemistry) with a lot of lab experience you can get a decent job, but someone else will be deciding how the research is going to be done. If you want to be helping to set direction in a research field, a PhD is likely to be needed.
For the PhD program where my daughter is currently studying, I am pretty sure that the majority of the PhD students do not have a master’s degree, but a significant minority do have a master’s. There are also a few students who applied for a PhD program but were accepted for the master’s degree program instead. Again if they choose to go for the master’s degree these students would be way better off if their parents could afford to help them pay for the master’s degree.