This seems to be the case with most dialects of Spanish, but wasn’t there a recent thread claiming that some Spanish speakers pronounce these letters differently? (Specifically, ‘y’ would be [j] while ‘ll’ would be [ʎ].)
The two phonemes used to be different, but the difference is disappearing in general, and has disappeared already in many places. In my phonetics class (9th grade Spanish), the book listed both phonemes; nobody in my year pronounced them differently (about 220 students) and the teacher needed to concentrate to do it. My mother, who trained as a teacher specializing in Humanities subjects, claims she can: she can, but she only does it when she’s thinking about it, most of the time her poyos (supports) sound exactly like her pollos (chickens).
Here’s a Wiki article on yeísmo, which addresses where the dialects that make the distinction are. I’m most familiar with it in Argentinian Spanish, where the “ll” becomes something like a “zh” sound.
The Argentinians sort of go the other way, tho, at least the ones I know: they pronounce pollo almost as posho, and apoyarse almost as aposharse; rather than distinguish the two phonemes they use a different one.
You know what? You’re totally right. I forgot that the “y” also gets shifted to the same phoneme as the “ll” – either /ʃ/ or /ʒ/. So, like you say, “y” and “ll” are the same sounds in those languages, although a different sound than is usual.
(As an aside, the same sort of thing happened in Hungarian. Except in certain dialects, “ly” and “j” are the same sound /j/, although historically, the same sort of distinction was made between them as between Spanish “ll” and “y.” So Hungarian also has undergone their own version of yeísmo.)
Thanks for the precision.