As long as my kids are dependents, I’ll pay for items I think are necessary. Phone bills (though I did make older S pay $65/month for his data plan from age 12-15 because I didn’t think it was needed. Cell phones weren’t as common then), car insurance, car maintenance/repairs, health costs, etc. I pay for some clothes usually at Xmas/birthdays, but most they buy themselves.
Re: school expenses. We have set aside ~3-3.5 years of tuition/room & board at an in-state public. They know any money leftover at graduation is theirs to keep. Both have done/did very well with scholarships, so they will have/had a significant mount leftover. So, it really doesn’t matter who pays for the books as it comes out of that account.
Any money earned from a part-time job is theirs to keep and spend (or save) as they see fit. They pay for all fun type of expenses, and as I mentioned most of their clothes. Both kids have started retirement accounts and investment accounts. They are much more savvy in that area than I am. And I confess that most months I do/did gift them $50 because I am pleased at how well they are doing (or did) in school and with their spending habits. I have made it clear that it’s not a given and they do/did thank me every month.
(Tense is confusing when you have one independent kid and one still in school! Independent kid is pretty much 100% independent. He’s still on my cell phone bill because I’m lazy. When he gets married, I’ll switch. And he’s on my dental/vision plans because I have to pay family rates anyway. He does have his own health insurance since ours would be out of network in his area)