https://i.imgur.com/o4fHpFX.mp4

Shader "Unlit/SpriteMask"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Mask ("Mask", 2D) = "white" {}
    }
    SubShader
    {
        Tags { 
            "RenderType"="Transparent"
            "RenderQueue" ="Transparent" }
        LOD 100

        Pass
        {
            ZWrite Off
            Blend SrcAlpha OneMinusSrcAlpha

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
                fixed4 color:COLOR;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                fixed4 color:COLOR;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;

            sampler2D _Mask;
            float4 _Mask_ST;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                o.color=v.color;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                
                fixed4 main_col = tex2D(_MainTex, i.uv);
                fixed4 mask_color=tex2D(_Mask,i.uv) ;

                // fixed4 res= main_col* (1-mask_color.z) + mask_color* mask_color.z * i.color; 
                fixed4 res= main_col* (1-mask_color.a) + mask_color* mask_color.a * i.color; 
              
                return res;
            }
            ENDCG
        }
    }
}