shader code to do equirectangular lookups on a texture

Equirectangular.shader

// https://forum.unity.com/threads/mapping-a-point-in-world-space-to-a-uv-of-a-sphere.760868/
// https://forum.unity.com/threads/what-is-wrong-with-unitys-spherical-mapping-how-to-fix-it.321205/#post-2083408
Shader "Custom/Equirectangular" {
    Properties {
        _Color ("Main Color", Color) = (1,1,1,1)
        _MainTex ("Diffuse (RGB) Alpha (A)", 2D) = "gray" {}
    }
 
    SubShader{
        Pass {
            Tags {"LightMode" = "Always"}
 
            CGPROGRAM
                #pragma vertex vert
                #pragma fragment frag
                #pragma fragmentoption ARB_precision_hint_fastest
                #pragma glsl
                #pragma target 3.0
 
                #include "UnityCG.cginc"
 
                struct appdata {
                   float4 vertex : POSITION;
                   float3 normal : NORMAL;
                };
 
                struct v2f
                {
                    float4    pos : SV_POSITION;
                    float3    normal : TEXCOORD0;
                };
 
                v2f vert (appdata v)
                {
                    v2f o;
                    o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                    o.normal = v.normal;
                    return o;
                }
 
                sampler2D _MainTex;
 
                #define PI 3.141592653589793
 
                inline float2 RadialCoords(float3 a_coords)
                {
                    float3 a_coords_n = normalize(a_coords);
                    float lon = atan2(a_coords_n.z, a_coords_n.x);
                    float lat = acos(a_coords_n.y);
                    float2 sphereCoords = float2(lon, lat) * (1.0 / PI);
                    return float2(sphereCoords.x * 0.5 + 0.5, 1 - sphereCoords.y);
                }
 
                float4 frag(v2f IN) : COLOR
                {
                    float2 equiUV = RadialCoords(IN.normal);
                    return tex2D(_MainTex, equiUV);
                }
            ENDCG
        }
    }
    FallBack "VertexLit"
}

Hey there, im sorry if this is a newbie question, but i am currently using this shader but i don't quite understand the math in RadialCoords(), do you have any reference on that? or maybe just a source for me to learn or a keyword to learn the method? thanks :)

that original post explains the issue,
https://forum.unity.com/threads/mapping-a-point-in-world-space-to-a-uv-of-a-sphere.760868/