What’s Inside Tool Batteries? Milwaukee FLEX DeWalt Makita Snap-On [18650 21700 ??]

Which battery cells does each tool manufacturer use in their tool batteries? We take a look under the hood of the RIDGID, Snap-On, FLEX, Milwaukee, Makita, and DeWalt batteries to find out. Which ones are 18650 cells and which are 21700, and what’s the difference? Is Samsung the go-to choice, or is it Sony, Panasonic, or Sanyo?