Although there were usual upgrades like battery life, SSD storage etc, the biggest upgrade was the M1 chip.
Apple’s M1 chip was a custom-designed system on a chip (SoC) that was first introduced in November 2020. It was based on the ARM architecture and is used in a number of Apple’s products, including the MacBook Air, MacBook Pro, and Mac mini. It also included an 8-core CPU and a 7- or 8-core GPU, which provide impressive performance for a wide range of tasks.
In addition to its performance, the M1 chip was also notable for its energy efficiency. It used less power than previous chips, which allowed devices equipped with it to have longer battery life. Apple claimed that the M1 chip could improve battery life by up to 20 hours on a single charge in some cases.