How to write a shader like this? [Screenshots]

Interested in how to implement the dirt on the model.

Now I have only this:

 Shader "Custom/DirtyShader" {
     Properties {
         _Color ("Color", Color) = (1,1,1,1)
         _MainTex ("Albedo (RGB)", 2D) = "white" {}
         _Glossiness ("Smoothness", Range(0,1)) = 0.5
         _Metallic ("Metallic", Range(0,1)) = 0.0
         _BumpMap("Normalmap", 2D) = "bump" {}
         _DirtMap("Dirt (Normalmap), Mask (Alpha)", 2D) = "white" {}
         _Dirtiness("Dirtiness", Range(0,0.9)) = 0
         _BumpHeight("Bump Height", Range(0,1)) = 1
         _DirtColor("Dirt Color", Color) = (0.39,0.23,0,0)
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         CGPROGRAM
         // Physically based Standard lighting model, and enable shadows on all light types
         #pragma surface surf Standard fullforwardshadows
 
         // Use shader model 3.0 target, to get nicer looking lighting
         #pragma target 3.0
 
         sampler2D _MainTex;
         sampler2D _DirtMap;
 
         struct Input {
             float2 uv_MainTex;
             float2 uv_DirtMap;
         };
 
         half _Glossiness;
         half _Metallic;
 
         float _Dirtiness;
 
         fixed4 _Color;
         fixed4 _DirtColor;
 
         void surf (Input IN, inout SurfaceOutputStandard o) {
             // Albedo comes from a texture tinted by color
             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
             fixed4 dec = tex2D(_DirtMap, IN.uv_DirtMap) * _DirtColor;
             o.Albedo = c.rgb;
             // Metallic and smoothness come from slider variables
             o.Metallic = _Metallic;
             o.Smoothness = _Glossiness;
             o.Alpha = c.a;
 
             clip(o.Alpha - _Dirtiness);
         }
         ENDCG
     }
     FallBack "Diffuse"
 }

Thanks for any help.