Question Adding Stencil Buffer to URP Shader

Hi,

I am new to shader coding, I have managed to get a basic shader to work with a stencil buffer as I need to for our project.

I have a stencil write shader, and simply read it within my stencil read shader as follows

[IntRange] _StencilRef ("Stencil Reference Value", Range(0,255)) = 0

...

Stencil {
Ref [_StencilRef]
Comp Equal
}

A couple of questions -

a) How would I get hold of the Unity Source for the URP shaders (currently using Lit for starters), so I can add my Stencil code to it? (I will just leave the rest of the code as is)
b) Is there another method to do this that I am not aware of?

Thanks for your help,
Matt