You do not need to pay interest. You need to let statements close out monthly with a balance. Interest on that balance will not accrue until the next cycle closes.

Say your statement closes on the 1st of the month. Buy something on Jan 15th. Let the statement close Feb 1st. Pay it off any time before Mar 1st. You get a benefit to your score by showing utilization without accruing any interest.

Paying off a loan hurts your score because of other things like reducing borrowing power, messing with utilization, and usually lowering the average age of all accounts. Nothing to do with paying interest.

If you can't jump straight to a car or home loan, collect credit cards if you have the willpower to avoid debt. That's how I built it up. I got a new card annually for several years. I'd take advantage of intro promos, but once those were up, I'd just assign some small monthly subscription to a card. Gotta spend and close with a balance to look good on paper, but also need to occasionally spend to keep them from canceling the card, too. At peak, my combined credit limit was more than my income, spread across 8 cards. That was double an average car msrp. It worked perfectly fine for a mortgage application despite driving older cars bought with cash.

You CC is like a long term loan. Just using a couple and paying them off every month got me up to an almost perfect score.

This is completely wrong. Your credit score is NOT how much interest you pay. It's how likely you are to repay your debts. You need to show that you will always pay, and never skip a payment.

My credit score for years was absolutely amazing, entirely on credit cards, and without paying a single penny of interest, ever. Instead, I paid in full every single month on every account.