Need ultra simple tutorial for using Walker_Nav_Menu in a block theme

Hi,

I spent hours on tutorials on how to make a custom nav walker.
But still don’t manage to do anything right with my theme (block theme).
Not even to change the class of the nav or the ul elements…

For exemple I don’t even understand where to find the ‘theme_location’ name (once I have created a menu in the WP appearence>editor)…

Also, I don’t understand what is the difference between the menu location, the menu name, and the navigation ‘ref’ number that I can see in my header.html template…

So, do you know where I can find an ultra simple step by step tutorial on how to build a Walker_Nav_Menu when using a block theme ?

Thank you!