The Transformers Legends manga are a series of short-story manga packaged with Japanese Transformers products, meant to tell wacky parody stories of the Transformers franchise. They're essentially a fun way to be an extra commercial since the stories feature the actual toys. The Legends manga also exists as a web comic.
Lore wise, the Legends Universe is sort-of like Space Jam and Roger Rabbit in concept. The setting is a fictional Japan where many people are Transformers fans, while also having the Transformers characters themselves coexisting as they visit from their respective universes. The main storyline revolves around the antics of the Beast Wars characters as the Maximals and Predacons operate rival trading companies. Each one has a specific favorite like Rattrap loving Beast Wars and Rhinox having passion only for G1. (This is where the actual figures of the toylines come in.)
While the Beast Wars cast are the 'main characters', other characters also take center stage. An interesting quirk of this universe is that there is something known as the Transformers virus that turns humans into the Transformers characters themselves, thus the 'universal counterparts' to the canon characters. A lot of classic anime/manga tropes are used and are spliced with Transformers lore.
Example: BW Rattrap lives with G1 Arcee, his great-aunt as mentioned in the original cartoon. However, in the Legends world Rattrap's auntie was a regular human who was turned into G1 Arcee by the Transformers virus. Because of this, she is portrayed as the loving and beautiful mom/aunt character who is very popular among the male characters.