The universe is one
The One Piece universe is a vibrant and vast world created by Eiichiro Oda. It centers on the adventures of a group of pirates led by monkey D. Luffy as they sail the high seas in search of the ultimate treasure, the One Piece. This treasure is said to be at the end of the Great Line, a dangerous road full of dangerous creatures, powerful enemies and unpredictable weather.
In this universe, some individuals have gained superhuman abilities by consuming mysterious fruits known as Devil Fruits. These fruits come in several varieties, each with unique powers, but instilling in users a common seawater weakness. The One Piece universe is filled with different locations, mysterious objects, and many characters, each with their own unique stories and designs.
The One Piece universe isn't all about exciting adventures and laughs; it also delves into deeper themes such as justice, freedom, dreams and friendship. It approaches the gray areas of morality and thus appeals to its readers/viewers on multiple levels.