Hi there,
You should be able to achieve that layout by setting the featured image to below title and align left:
https://docs.generatepress.com/article/adjusting-the-featured-images/
You will likely have to crop the image so it’s small enough to have text on the left side.
As for changing the featured image in single posts, looks like the example site is simply hiding the original featured image in the single posts and insert a new image at the start of the content.
Let me know if this helps.