when brittish colonists settled in the americas they no longer had to preserve biscuits for storage or sea voyages so instead baked them once and left them soft, often with buttermilk or whey to convert cheap staples/byproducts into filling items to bulk out the meal to make a small amount of greasy meat feed a whole family. considering hardtack biscuits were typically eaten by dipping them in grease or gravy untill they became soft enough to eat without breaking a tooth this was a pretty short leap of ‘just dont make them rock hard if im not baking for the army’ but didnt drop the name because its been used for centuries and people forgot its french for ‘twice baked’ back in the tudor era, biscuit was just a lump of cooked dough that wasnt leavened bread as far as they cared
thus the buttermilk biscuit and the hardtack biscuit existed at the same time. ‘cookies’ then came to america via german and dutch immigrants as tiny cakes made with butter, sugar/molasses, and eggs before ‘tea biscuits’ as england knew them due to the new availability of cheap sugar- which is why ‘biscuit’ and ‘cookie’ are separate items in america but the same item in the UK
the evolution of the biscuit has forks on its family tree