There was a story about 2 brothers who had a natural talent for alchemy called Fullmetal Alchemist. This story was originally told in a manga, but before the manga was finished an anime was put out. The anime had to change and create some original story elements because the manga was still ongoing at the time.

After the manga had finished they wanted to revisit the anime with a version closer to the actual story told in the manga. This would be known as Fullmetal Alchemist: Brotherhood. The only thing was due to retreading some story elements told in the original anime, they shortened some story beats for FMAB.
Intention:
Online fans had come up with ways to watch some episodes of each mix and match FMA and FMAB to get the full story. One of these was told on Anime Planet here: https://www.anime-planet.com/users/Halex/lists/fullmetal-alchemist--watching-guide-191736

So I figured why not create replacement episodes for FMAB by combining the required viewing from FMA into longer episodes to create an actual manga cut for rewatching based on the Anime Planet article.