The majority of anime series are adapted from manga, and in most cases, the anime does a good job of following its source material. There are times when an anime airs while its manga is still ongoing, and this usually causes the anime to introduce filler episodes. In certain cases, the anime will tell an original story instead. Fortunately, there are several great anime series that can be watched because their respective manga have now ended.
Source：https://www.cbr.com/anime-with-finished-manga/
