Apple IPhone System Fonts

Hello! Could someone please tell me how I use/change the font that is used on my iPhone 11 XR?I currently use the font that was on the phone when my husband gave it to me.(which I’m assuming is the stock one that is downloaded on all iPhones.”) To look at the fonts I go to <General~Keyboard~Fonts, but once I’m there it doesn’t show how me to use the different ones, maybe in text messages or in social media. Someone PLEASE tell me how to do this?! Thank you! Everyone have a spectacular weekend!