Here's the Broadcom BCM2042A4KFBGH chip that allows the Magic Mouse to talk to its host.

This is a BCM2042 "Advanced Wireless Keyboard/Mouse Bluetooth Chip."

According to Broadcom, "By integrating all components within today's mouse and keyboard into the BCM2042, low system costs can be achieved to approach the price points of legacy-wired mice and keyboards." Apparently Apple missed that memo.