What made the biggest impression on my kids was revealing the bills to them. Instead of just paying the phone bill, I opened it with them and made them tell me how much the basic cell phone line cost, how much data for a smartphone cost, etc. Instead of just paying the electric bill, I showed them the bill. Same with the water, the trash, the mortgage, cable tv, car insurance -all the things that directly involved them since they lived at the house and used the water and electricity, etc.
When they saw the amounts, it dawned on them. And when it was time to get them car insurance, I made sure my phone was on speaker mode and my teen was sitting right there and participating. They heard the questions that were asked (had they taken drivers ed, were their grades excellent, etc), and then together we heard the choices for insurance limits, deductibles, etc. I made sure that they understood deductibles, and other insurance terms, and that they knew who their insurer was, and how to contact them in case of emergency or accident. I was shocked at how many of my son's friends didn't even know the name of their car insurance company.
I think many parents tell their kids "this stuff costs a lot of money" but until the kids see it in black and white, they don't have a reference point. And although they'll be paying for things differently than we used to, sometimes, they still may need to write a check for a deposit on an apartment, or at a business that doesn't have Apple pay or an electronic pay method. I think they need to learn the basics of check-writing and check receiving and account balancing.
We paid for basics, but made my son pay for gas and registration. As long as insurance was kept to a minimum due to no accidents or violations, we paid for that at first. We paid the basic phone plan, but if they wanted smartphones, they had to pay the extra cost involved with that (data plan). And we pretty much monitored how his behavior about money was going during those first few years of semi-independence. Was he losing things quickly (How did that new phone case that was so pricey "disappeared" after a week? Did he not know where the expensive calculator was before the first semester was over? Did he run out of gas? Did he take care of the car?) Since he proved to be responsible and trustworthy, we were a little more lenient. But if he had been irresponsible, we would have been much more strict.
So, I guess my basic answer about what is most important is: Knowledge. Being involved in the money paying process. Information.