The chip is different: The apple watch7 uses a dual-core S7 SiP chip with a 64-bit dual-core processor and also has GPS, compass, all-weather altimeter, second-generation speaker and microphone capabilities.
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The apple watch series 8 features an S8 chip with a 64-bit dual-core processor, a built-in lithium-ion rechargeable battery for up to 18 hours, and a USB-C magnetic fast charging cable.
The difference is that the operating system is different and the chip is different, the chip used by Apple S7 is the t6 version of the chip and S8, the chip used is the t9 version of the chip, the S9 chip runs faster, faster, very fast