Not all manga are destined for anime. It's a complex decision based on multiple elements. For example, if a manga doesn't have a wide enough fan base or if the production costs are too high, it might not get turned into an anime.
No way! Anime can be made from all sorts of things. Sometimes they're the brainchild of the animation studio, sometimes they're based on games or other forms of literature. Manga is just one of the many starting points for anime.
No, not necessarily. Anime can be based on various sources. Some are inspired by novels, some by visual novels, and others are completely original concepts developed by the animation studios.
Not all anime are based exactly on the manga. Sometimes, there are changes made for various reasons like creative decisions or to fit the animation format better.