I have a question. Why the /j/ in "university" /ju:nɪ'vɜ:sətɪ/ is pronunced "J" but the /j/ in "onion" /'ʌnjən/ is pronunced "i"? Why "onion" is not /'ʌnɪən/?
Not sure if you had people answer you clearly.
/j/ is a semi-vowel. It's produced from the tip of the tongue, but it has the characteristic of a consonant.
/i/ is exactly a full vowel--giving an extra syllable.
When you pronounce /ju:nɪ'vɜ:sətɪ/, the tip of the tongue slides from /i/ position to /u/ position.
Some old dictionary used to give the pronunciation with /'ʌnjən/ because of the blend of /i/ and /ə/. However, if you give the pronunciation of /'ʌnɪən/, the word is divided into 3 syllables, rather than 2.
If you study the RP, you will find out that some speakers distinguish the /i/, /I/ and /i:/.
The IPA uses /i/ in diphthongs, such as /iә/ or /aiә/. I can't recall most of the excerpt from those books, but you might want to search publications by Oxford or Cambridge, esp. those older than 1990s.
Nowadays many RP speakers only pronounce /i:/ and /I/.
Oh, and I forget. Words in plural, such as cases or passes, are used to be represented as /'keisIz/ and /'pɑ:sIz/ for the RP, but for Americans, they use the /'keisәz/ and /'pæ:sәz/