It was P2P as it used file hashes to look for other clients to share the file so you didn’t need to rely on downloading from specific users directly like napster, but the other features depended on when you used it and what client. Originally, it was centralized and wasn’t that different than its contemporaries in how you used it, but then an improved client was released (eMule) and it added support for a second decentralized network (KAD) and it also used compression and had a bunch of better features like robust bad IP blocking (RIAA was ramping up their bullshit around then) and way to disguise the traffic to prevent ISP snooping/blocking.

Not sure about anyone else, but I used a website with eDonkey links (which also worked in the Overnet client)