iPhone6s full storage and other full

currently I have an iPhone6s and I have 16GB I have one app currently downloaded because my storage says it’s full even though the apps I have don’t add up to 16GB and my other storage says it’s sometimes at 7 or 8 gb and it’s taking up most of my storage and I try doing stuff that I see on this page but I don’t have a pc or mac and since were in quarantine I can’t go out really and I don’t know what to do and I need to update my ios to the newer version so that hopefully it can fix the issue but once again it says I need more data and I don’t really want to spend money and my messages get glitchy and take away the photos and doesn’t let me send back I message games takes me out of the whole app ..