Creating a scratch card effect in Unity for mobile

Hey guys,
I’m trying to create a scratch card effect in Unity, where I have two images: the first one is the image to be revealed by scratching, which will be underneath the second image. The second one is just a grey image.
I’d like to write a script that “scratches off” parts of the grey image where it is touched on the mobile screen. Only the parts that were scratched off (touched) should reveal the image underneath.
I’d also like to keep track of how much of the image has been revealed. If most of it has, I want to remove the grey image completely.

I’ve tried searching for a solution in many places but none of the answers were sufficient.
Please note that I’m a beginner so answers with as many details as you can provide would be greatly appreciated. Also, I do not wish to use an asset from the asset store.
Thanks!

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