Before that, I had an Amazfit Bip for a few years which was incredible. It did notification, GPS, heart rate tracking, always on display and battery life of 2 - 4 weeks. It did this years and years ago, when the best Android could do was 24 - 48 hours, and it did it for like £60 instead of £200. It still works too!
I don't know about Amazfit, but I have a Garmin that also lasts weeks. There are some differences: WearOS/WatchOS watches essentially use a more power-efficient/less powerful version of a smartphone-class SoC. They have to because they run a full Linux/XNU kernel and a pretty complete userland. Watches with weeks-long battery life typically use something that is more akin to a powerful microcontroller with operating systems tailored to such low-end hardware.
Besides that some watches (like several Garmin models) use transflective displays. They do not have to actively emit light during daytime (in contrast to OLED), sunlight is reflected. In contrast, OLED displays have to be more bright in sunlight to be visible.