Ok...I'll ignore the information from the credit courses I've taken
You got ripped off if the credit course told you to leave a balance on a card month to month.
A lot of people misinterpret the how reporting actually works. If you’re using your card all through the month, at some point the card will report whatever the balance is to the credit bureau. Whatever the balance is on that day would be your balance for the month. So your use of the card is still showing up on the report. That balance still gets reported even if you pay the card off the next day. Which is how you build credit and avoid paying interest.
A lot of people confuse this advice with randomly carrying a balance and paying interest on it.
To put this into an example. I use a Chase Card got all my purchases. We’ll say I spend about $1600/month on my card or $400/week. We’ll say my bill is due on the 1st and the reporting date/statement date is the 15th. We get to the 1st and I pay all $1600. I continue using it and by the 15th, I’ve put 800 on the card. So my bill for the month so far is $800. Chase sends a report to the credit bureaus saying I’ve spent $800 on the card. I continue using the card as normal and pay it off on the 1st like normal. My credit report still shows I put $800 on the card for this month even though I paid the whole bill on the first of the next month.