Yes. maybe I can help you. If you click once on the hard drive's icon and press cloverleaf (apple key) and I, it'll tell you info about your hard drive, including its capacity. These days, I think, rather than using a traditional hard drive, Apple uses a "Blade SSD"-which is supposed to be faster to access stuff, but kinda limited in overall capacity, and more expensive. It used to be that if you wanted a hard drive with more space on it, you'd shut your MacBook down, back up your stuff, crack 'er open and put in a hard drive with bigger capacity. Those days, sadly, are long gone. The easiest way to do it, and which is free and built into the OS is Time Machine. You'll need an external drive with roughly the same capacity as the one built into your MacBook Air. You plug it via USB or Ethernet. If not Mac formatted, you'll have to format it, which is pretty easy to do. Then after you have formatted your hard drive and plugged it in, you turn on Time Machine, tell it which drive you want to use as a backup, and then let it start backing up your MacBook pro, and it will do so. It's a good idea to have a backup in place, no matter what kind of Mac you have, as hard drives can and do fail, often without warning. Not trying to scare you...just...sayin'... You can also schedule it to backup when you want it to, and it'll do it automatically and securely. IF you find that your hard drive is getting too full too fast, you might want to consider investing in a bigger hard drive. I don't know if your local Apple place would install it for you, they might, but you'd have to check. I'd suggest going to macsales.com to see what options are available for your MacBook Air 13". I don't know exactly what year of MacBook air you have, so I have to go with what you have given me. yes, if you have stuff in iCloud, it means that it's still taking up space, unless you have deleted it after that.
good luck to you
john b