The page selection is set within the Customizer under Theme Options, which will only show when front page is set to static.
The output happens within the function twentyseventeen_front_page_section in the theme file template-tags.php, where each panel is queried individually with get_post, so it’s not exactly a loop in the conventional sense.
You can change the number of sections from the default 4 with the twentyseventeen_front_page_sections filter, which should return the number of desired sections.
To access the values via code, you can use get_theme_mod, each panel is stored under the key panel_$i, where $i is the section number, starting at 1. The stored value is the ID of the page. You can also update these values with set_theme_mod.