.package file byte layout

No idea if this will help you, but SimGuruModSquad has information. I think they publish a Maxis guide regularly and unless I'm mistaken, all of the Sims 4 employees are known as "simguru" on the forum. You can take a look at this page, too, to see if there's anything useful. Some more maybe helpful links here, here, and here. I realize these links are all specific to the game and not Studio but there's got to be some crossover I would think. andrew is the one to ask specific questions about how Studio works. You can also see what's on Mod the Sims. They have a lot of Sims 3 tutorials and Sims 3 used package files (and sims3packs which can be unpacked into a package file). Some of the older tutorials on their wiki are gone but you can still find them on Wayback Machine. TSR Workshop used to have Sims 3 tutorials that you can find here. They have a blog currently but I think the tutorials are all geared toward gameplay. 

Hope this helps. If not, well I tried, lol.