Single content title is the post title/page title of a post/page which generated automatically by WP.
H1 is for all H1 elements (manually added and automatically generated) including single content title.
For example, if you set font size for H1 to 40px, then all the H1 would be 40px.
But if you set font size for single content title H1 to 20px, then the single content title H1 will change to 20px, other H1 elements keep the 40px.
Archive title H2 is the post titles on blog/archive page generated by WP.
The relationship between H2 and archive title H2 is similar as the H1s.