Use first post image when featured image is missin

I’m trying to replace some older WP themes where I have made customizations with php + css and I’m getting used to really like full site editing.

Many things are solved without me putting in any code but one thing remain.

In a grid I’m trying to get it to use the first image from the post when a featured image is missing. I know there are plugins that allow me to take the first image and put it as featured image but I would really like not to do that. It’t thousands of posts and Im not keen in doing any auto-magics-stuff on that many posts.

And also if there are none I would like to have a colored block or something for the symetry.

I heave no idea even where to start. Any tips?

Thanks!