Differences between ttf folder and noto-set folder in more detail

When I download opensans-main.zip, I find two folders: Once the ttf folder, then the noto-set folder, whose fonts contain almost five times the number of glyphs as those from the ttf folder. Except for the clear difference in glyphs, I basically don’t understand what the difference is between these fonts, or what the second folder has to do with the noto font. Is the latter just a derivative of Noto and not a “real” Open Sans? Can someone explain me the differences in more detail? Thank you.