The British and Americans disagree over the meaning of cookies & biscuits. However both words come from different traditions in the British Isles and both are correct. Some so-called American words actually come from England.
Originally biscuits were small, round, thin, flat, crispy, hard, dry, unleavened bread. The word biscuit is from Old French and meant twice baked. The main ingredients were flour and water. Hard tack, or pilot bread in the US, would be the original biscuit. They could be kept for years as an emergency food.
The word cookie is from Scots English (Or possibly from Dutch into Scots, then into common English) for a small cake. These cookies became the soft, sweet and chewy American cookie, which is now a kind of miniature cake. Over time sweet and fancy versions of cookies and biscuits were invented and are now made with butter or shortening, sugar, milk and flour.
The traditional British biscuits, such as shortbread, are dry and crispy. So they are distantly related to the original meaning of the word biscuit. They can be stored longer, but I wonder why anyone would want to eat one. The American cookies are related to the original Scottish cookie meaning soft baker's bun, but not the English meaning for hard dry biscuit.
Today Britain uses the word biscuit and America uses the word cookie for their dessert versions. Just to be confusing, each country uses the other word, but for different things. In America a biscuit is a baker's plain bun, similar to a scone. A cookie in England is often an American-made soft cookie, that is so-labelled by the company. In Scotland, a cookie originally meant a baker's plain bun, but English and American use also exists.
There are two words being used, cookie and biscuit. There are two cookie traditions; some people prefer dry crispy cookies and others prefer soft chewy cookies. It would be easier if those dry crispy cookies were called British biscuits and the soft ones were called American cookies. But no! Both countries confuse the meanings of these two words and types of cookie/biscuit. To summarise, in England a biscuit is a small sweet dry bread. In America a cookie is a small flat cake and a biscuit is a bun. When we use either of these words the meaning must be explained directly or in context.