Dear David: Personal Finance and Life Letters from a Grandfather to a Grandson

What do you do when your high school senior grandson asks in a nighttime text:

Hey Granddaddy, so I know you're probably asleep by now but I just wanted to ask what you think about money. What would be the best way to make the most over a "long" (like 10-20ish years) period of time? Savings, investing, side business[es]? Going into the military, having a long period of time being able to save or invest, or build a good business to be able to gain the most amount of money possible?

And then buying. What is the best way of buying things (like houses and cars...big things), in cash at once or like over a period of time (even though there's interest)?

Split it up cause there's different parts. Thank you lots. Love you!