Open credit raises your credit score. What that means is having a credit card you don’t use will make your score higher, or if you do use it, using very little of available credit and paying on time, and you should pay in full so you don’t get gouged with high interest. Never charge something unless you actually have the money.
There is a MYTH that if you pay in full your credit score will be lower. My husband and I always pay in full (always as in over 30 years) and his score is always above 800. Mine is usually slightly below or above 800 because I have more medical bills and healthcare is a nightmare and pulls my score a little down once in a while.
So, for example your credit card has a limit of $5,000 and you charge $300. Your open credit is $4,700 and then you pay in full and it goes back to $5,000. Even if you pay in full every month, when they run your credit if the $300 is on there at that moment it will show as money you owe
By you not using all available credit it’s demonstrating you control yourself, and don’t just spend wildly just because it’s given to you.
I’ve never heard of a security deposit for a credit card. What credit card is it? Are you sure it’s a credit card and not a debit card? A debit card does nothing to help your credit score.