Okay, as an Australian who lives in the US now, let me see if I can sort out the confusion for everyone.
Commonwealth scone is approximately American biscuit. In the US, they are usually eaten with savoury things, kind of taking the place of a dinner roll. They're eaten buttered with dinner, or covered in gravy. In the Commonwealth, they are usually eaten with sweet things, for afternoon tea for instance, with cream and jam. The recipes may differ slightly, but the concept is the same - light, fluffy bland-tasting thing used as a vehicle to deliver yummier things to your mouth.
https://thebarking.com/wp-content/uploads/2012/09/biscuit.jpg
American scone doesn't really have a Commonwealth equivalent. It's kind of like a larger, denser Commonwealth scone, more cake-like than light and fluffy, with fruit on the inside, sometimes sugar on the outside, and cooked so that the outside is harder than the inside.
https://blogs.babble.com/family-kitchen/files/2011/03/Blueberry-Scones1.jpg
Commonwealth biscuit is the same as American cookie. In Australia we use cookie as well, specifically to mean the choc-chip cookie-style cookie. Anything that's more shortbread like, or cream- or jam-filled (like the picture below), is a biscuit (or even a bikkie) - I'm not sure if that's the same in the UK.
https://www.deweys.com/managed/bakery-cookies.jpg
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