What’s hidden, except in texts which use quantity marks, is the quantity of the vowel that follows the i: long u in Iulia, long a in ianua, short a in iam.
Older texts used j for this consonantal i, convenient but wrong.
As to initial ie, ia, and io, yes the i is consonantal except in a few words (mostly Greek names) where it’s a regular vowel. You won’t find iy, since y is not a Latin letter.