Moodle does not use html file, but mustache templates which are 'fed' by the layout php files, with many components renderered in Output lib files and classes. The course overview for example, though is a block which is put onto the dashboard page and could be tweaked by cloning and customising the block itself, or through the theme, depending on exactly what you are trying to achieve.
Howard's recommended link is strongly recommended as a starting point, but then depends on exactly what you are trying to change on each element and whether it would be achievable in mustche templates, layout files, renderers or even the actual block itself.
Howard's recommended link is strongly recommended as a starting point, but then depends on exactly what you are trying to change on each element and whether it would be achievable in mustche templates, layout files, renderers or even the actual block itself.