I needed to install .NET Framework 4.7.1 Developer Pack to get intellisense and autocompletion to work correctly in Visual studio Code.
Hopefully this helps anyone in a similar situation.
Extra information, when following this tutorial and writing this message i was using
Unity 2020.3.32f1 and Visual studio code 1.66.0, both newly installed onto a clean laptop.
with extensions “C#” and “Unity code snippets” enabled, as per the tutorial suggested.
p.s. I also closed up unity, unity hub and code completely while installing this pack which no-doubt is advisable.