Views theming
Views is very complex and is made up of many pluggable layers. A View has a display (such as a Page or Block), which can render its content using a given style (such as an Unformatted list or Table). Styles can decide whether to control the rendering of a given result item (row) themselves or delegate this to a row plugin (such as Fields or Entity). Most, in fact, do the latter. The two most common scenarios for using row plugins is either using the EntityRow one, which renders the resulting entities using a specified view mode, or the Fields plugin, which uses individual ViewField plugins to render each field that is added to the View.
If we wanted to theme a View, there are all these points we can look at. Want the View to output a slideshow? Perhaps create a new style plugin? Want to do something crazy with each entity in the result set? Maybe create a new row plugin, or even just create a new field plugin (as we did) to render one piece of data in any way you want...