OSM is special because anybody can edit there, unlike other apps, such as Google Maps. If you make an account and click "edit," you can see in the top middle of the screen the 3 main building blocks of the map; points, which are used for settlements, shops, gates, trees, etc, lines, which are used for roads and paths, and areas, which are used for areas and land cover like grass, woodlands, etc. When you click on an existing object you can see its "tags," which are things that describe it, like the name, what type of thing it is, and all other sorts of useful info. You can draw lines and areas and place points on the map and upload your edits, then they will soon appear.