Donno if it will help, but:
1) The only "technical manual" is Wiki, there is alot of basic info there. Sometimes hard to find though.
https://docs.larian.game/Main_Page2) You need TrasureTables, in StatsEditor. But to understand how whole process is actually working, you may need some more investigations. Like understang the item generation process.
3) Scaling from different stats ussaly made with scripts and/or new statuses.
For example, if you want to deal more damage depending on intelligence with scoundrel, you can:
a) make some extra dmg status apply on hit (will deal dmg immidiately), and setup this status to be scaled with INT (actually it will be damage_status with a _weapon attached to it.You can setup it the way you want)
b) if you dont like to have 2 different numbers at 1 hit (or plan to scale some buff, lets say, +dodge%), you just script it.
There are different ways to do it, depending on situation and the scripts you use (Story or .charScript), sometimes it is a pain ^^ but it is possible.
If you need some exact info - you'll have to dig for it, use search. Or make your own investigations ~80% of modding process, hehe.
Or you can try visiting discord server, ppl are answering there much faster, if questions are not too silly
https://discord.gg/mpxcPuV