Usenet was a distributed set of news peering relationships, exchanging posts via NNTP. There was nothing to acquire, so that's not what happened.
Google merely set up a web interface to it, which was eventually extended to the abomination that was Google Groups. This was intended to be some sort of mix of Usenet group and mailing list, all via a web interface. In reality it was awful and I suspect development on it stopped 10+ years ago.