The Attack On Titan manga was first released in 2009, but even with over a decade to conclude everything, there's too much to delve into to cover in the main story. Because of this, various spinoffs have been released over the last decade, some of which flesh out fan-favorite characters like Levi, Mikasa, and Annie. Others introduce viewers to the character who created the ODM gear, further fleshing out the lore of an already expansive world.
Source：https://www.cbr.com/attack-on-titan-all-spinoffs-ranked/
