The symbols [i̯] and [u̯] always confused me, like what makes them different from [j] and [w]?
There's not necessarily any difference.
One of the weaknesses of the IPA is that it considers consonants and vowels totally separate things and describes them with totally separate parameters. This means that
[j] is palatal, while
[i] is high front, and those two things aren't really connected in any way.
But of course, the articulation of a palatal approximant is nigh identical to the articulation of a high front vowel. So whether you choose to transcribe it as
[j] just depends on what you want to emphasize. If you want to get precise about just how high it is, symbols like
[e̯] are generally clearer than stacking diacritics on
[j], but if it patterns completely differently from
[i] (like in English), you might prefer a separate symbol to emphasize that difference. In a broad transcription, people might just use the one that's easier to type or easier to read.
Phonemically, the difference is clearer:
/u̯/ are generally used to emphasize a connection to
/w/ do the opposite.