HMD Global explains processes of How Android updates are achieved using a well elaborated infograph.

Nokia phones are quite popular for a clutter free Android experience (no bloatwares) and are also known for receiving timely updates on the latest Android versions. Despite all this accolades, it still takes a certain period of time for these updates to be rightfully achieved.
The operating system would be tested and forked to suit specific needs of various devices.
An example would be the Nokia 8 Sirocco, despite Nokia's trademark for timeliness they were delayed their slice of the Android 9 Pie.

It starts with Google issuing the latest OS versions to Chip vendors so they can make the needed drivers to ensure apt compatibility across the devices. The next step is where the System chip makers send Hardware Abstraction Layer to the Original Equipment Manufacturers so they can build the OS for specific devices. Also, the chip makers are to decide on devices that are eligible for support of the latest android versions.

Secondly, Nokia runs pure and "undiluted" Android one, which literally means that there isn't any additional tweak on the OS or forks like hiOS by Tecno, XUI by Infinix or MIUI by Xiaomi.
Internal testing would then be conducted to check compatibly across operators around the world. The next stage involves the Nokia community where a beta version is shipped to beta testers worldwide for critical feedbacks and future bug fixes and thereafter launch out an official stable version to eligible users.

Later tests would include conformance standard tests for Bluetooth, WLAN and other quality tests. After all the processes have been completed, the users then receives an over-the-air popup update if their devices qualify.

Source: https://www.gklited.com/2019/01/an-infographic-explanation-of-how.html?m=1