The "SBC" in SBCGlobal stands for Southwestern Bell Corp.
AT&T split into smaller divisions in 1984.
Various entities of Southwestern Bell Corp (SBC) owned these smaller divisions (aka Baby Bells).
One of these SBC Baby Bells, SBCGlobal, bought AT&T.
Yahoo connects to all this because AT&T (owned by SBCGlobal) and Yahoo created a partnership in 2008 to provide AT&T customers Internet services, such as email, news, forums, and these AT&T customers could access these services via a special Yahoo portal.
You have to remember at the millennium, AT&T was just a stodgy telephone company. AT&T didn't have the branding or recognition from these new, hipster Internet users. AT&T tried to attract these new customers with Google via my.att.net but then switched to Yahoo.
So, let's say in the early 2000s, you had Internet service with AT&T. Did AT&T have the infrastructure to provide email, news, forums, etc to its customers? No. How do we attract Internet customers? Partner with either Google, Yahoo, AOL, etc.
Pacific Bell was a Baby Bell. SBCGlobal owned Pacific Bell. Pacific Bell was on its Mommy's network: SBCGlobal.net.
Clear as mud?