A Phone has some service that periodically checks manufacturers servers (with some sort of polling) then if there is a new update it informs the user with a notification. User than clicks the notification and downloads and installs update from manufacturers server
For Example:
OEM create updates and FIX the Bugs in Existing OS and sent to OTA WEB MANAGEMENT SERVER and that communicate with OTA Delivery server, then our Mobile or system communicating with Delivery Server ( Over The Air ), Once we received the update from OEM we can download and Install it in our Machine.
OEM ( Original Equipment Manufacturer )
OTA ( Over The Air )
FOTA ( Firmware Over The Air )
If u want Brief Decription about this https://www.hatchmfg.com/explaining-ota-updates-for-android/