One word: The Internet. The World Wide Web offers: game FAQs; lists of cheat codes; gameplay tips & tricks; tutorials on skin-making, mesh-making and object-making down to the messy business of the Z-buffers; explanations of the file types used; analyses of the file contents; customizing the game, and what to do when it crashes. There is simply too much information available to list. Pages that I found informative are included in the Sim links, they should provide a good starting point.
Apart from websites, there are newsgroups and similar initiatives. While never having heard of a Sims newsgroup, I have pulled one very handy sheet of programming information from the "Files" directory of a Yahoo group called "Simfully". Newsgroups as a whole seem to have been replaced by forums (because Usenet was being flooded by spam?) and there are skinning forums, object hacking forums, where-can-I-find forums and doubtless many others. Unlike newsgroups, Yahoo groups and mailing lists, these can be browsed without having to become a member.
The official Sims websites are also a good source of information. Both the Maxis website (for PC) and the Sims version of the Aspyr website (for Mac) have a tips section and a helpdesk to email. Emailing the Maxis helpdesk requires logging in with an account, which can be had with a valid serial number. And of course they don't want their users getting too adventurous and giving advice that may ruin other users' installations and bring on a shower of complaints.
And then of course there's the manuals that come with each game/pack, for those who haven't lost them. I'd rather not lose them; they've got the serial numbers printed on their backs.
Finally, some info is within the IFF files themselves. It's not
likely to be very helpful, but the hidden comments in the CTSS block
(catalogue section) of buyable objects may explain their use and/or
origin. In these comments, accessible only with a hex or IFF editor,
I've found trivia like why the pool table is called "AristoScratch".