They bought Dejanews which brought in a large chunk of what was available on usenet.

But there was also a team inside Google that was specifically tasked with hunting ancient newsgroup archives from where ever they could find it, and a lot of people on the net contributed what they had archived.

For example some people physically mailed them stacks of CD's in cardboard boxes.

Kudos to that team for merging the whole mess into a usable, so far ever-lasting resource.

He made tape backups at his sysadmin job and those tape backups eventually migrated into bigger and more organized online archives.

(Not every old usenet post is due to his backups but many are.)