watchOS only runs on the Apple Watch and launched alongside the product back in 2015. It has seen several major revisions since its launch, such as axing glances, the friends UI, and more.
The original version of the software was very slow and relied on your iPhone for its apps. Everything presumably ran over Bluetooth, and would be unable to open any apps if your connected iPhone was out of range.
Apple quickly moved away from interaction model and pushed native applications as of watchOS 2.0, where the majority of the app’s interface and information were installed on your Apple Watch, instead of your iPhone.
watchOS 3.0 added support for background app refresh, which allowed apps to run in the background, but also enabled complications to be updated without needing to completely launch an app.
With watchOS 4.0, and the Apple Watch Series 3 with LTE, Apple took this interaction model a step further. The company introduced APIs to allow applications to run completely independently of the iPhone. This meant that users were allowed to stream Apple Music, get the latest News, or take a phone call without being in the range of your iPhone. However, an iPhone is still required to setup and manage your Apple Watch.
Learn more about watchOS by reading the articles below: