Close, it’s 18650 cells which are super common. Even stuff like laptop and EV batteries may be composed of them.
Fun fact, the numbers indicate the physical size - 18mm diameter 65.0mm length. The same applies to those button cell batteries - CR2032 is 20mm diameter by 3.2mm thickness.
The logic was just that when UNIX was originally evolving, they ran out of disk space on their PDP-11 and had to start moving less-essential binaries to a different disk. That’s why it’s “/usr/” which was originally for user data but that disk happened to have free space.
Any other explanation is just retcon. Some distros try to simplify things.