I just got my first Fico 8 score. What now?

What's the best way I can build up my credit score? I was looking to finance a used car but due to me having to also pay my unemployed mother's bills, I can't afford to comfortably add too much more to my monthly expenses while on $12 an hour.

My score was 746 but it dropped to 734 after I opened up a new credit line with Discover because my Capital One card only has a $300 limit and I heard having multiple cards helps build credit.

I'm hoping by time my mother gets back on her feet or by time I can get a better job, I can have a Fico 8 score close to 800 so I have better luck getting approved for a car. Do I just keep using by credit card has a debit card and pay it off before the monthly statement comes out or should I do something else? Would using financing services for phones or other things from online shopping improve my credit score after I get them paid off?

Also one other question. I have Chime and a week or two before I got my first Fico 8 score, I got the Chime credit builder card. Should I keep it even though it has no benefits or should I go ahead and close it to get a different card that has benefits?