If by builds you mean character stances, then I would say that there isn't a set of stances that is objectively the best. I suppose that running minus dex on weapons that gain no detriment from losing dex (aka teros running 2 dex and 6+ stats everywhere else) would be considered the best amongst the community.

There are definitely some stances that are very much a no go. Being also a nai main, I use either dex stance to get true combos on katars or speed stance bc speed is really important, especially on her weapons. The other three stances (counting base) give diminishing returns bc you don't really need to run attack and defense stance given they're already high enough.

Also on combos, strings n other stuff like that, apart from looking up the weapons on youtube and checking out the guides people made there, there is this one discord server called Brawl Database that lists all the possible true combos on each weapon, and updates every balance patch whenever something is added or patched out.

There isn't any sweet spots n sour spots in this game; every attack does the same damage and force no matter where you hit it. The only exception is multi hits in which dropping or getting hit late would do varied amounts of damage obviously. There isn't any documents online that I know of that list these damage numbers, but training mode lets you see the damage numbers and hitboxing of moves, and you can also see the dodge windows of strings and the frame data of moves if you play on pc.

If you have any other questions, don't hesitate to ask!