This article is about macOS High Sierra – but the newer versions are working the same way.
There is a way to install newer versions of macOS on older unsupported Macs. The tool is called „macOS High Sierra Patcher“ and can be found on this website with instructions:
Personally, I did it on my Mac Pro ’08 (MacPro3,1), which officially supports only OS X 10.11 „El Capitan“. With the „macOS High Sierra Patcher“ I managed to install the latest macOS (10.13). Now I have Siri and many other new features on this computer. The usage is still very good.