Do you even need to use Power Query Functions?
You can do amazing things in Power Query! Maybe you think to create a function. But do you really need a function for that? Or is it over complicating things?
Power Query Functions Overview
The Power Query M function reference includes articles for each of the over 700 functions. These reference articles are auto-generated from in-product help.
https://learn.microsoft.com/en-us/powerquery-m/power-query-m-function-reference
Power Query Custom Functions
If you're familiar with using Excel functions (and of course you should be) then you'll know that a function typically takes some input and gives you an output.
https://www.myonlinetraininghub.com/power-query-custom-functions
How to Uncover the Hidden Power Query Function List!
Hidden inside both Excel and Power BI is a secret Power Query function list that you can bring up while working in the Power Query editor window…that’s if you know the magic word.
https://www.youtube.com/watch?v=5a0dlTAEr2E
Parameters and Functions in Power BI / Power Query–Custom Functions
Power Query has over 600 native functions and the Power Query team keeps adding more and more.
I wouldn’t recommend memorizing them, but you do need to understand the concept of parameters and arguments in order to understand what functions are.