How has USB File Transfer changed in Android changed over the years?

*In Windows* I remember many years ago when I got my first android that USB connection seemed to work more like a flash drive. You could do multiple things at once without it saying the device was busy. In all newer android versions I've seen I get an error saying the device is busy if I'm copying something and trying to browse files at the same time. Also I've noticed newer android devices seem to be slower with file transfers or even generating all the files in a folder. So my question is what changed exactly and why was it implemented? *Bonus question: Can you give a detailed history on what happened with android USB and with what versions of android did it change with?*