No, it’s not complicated at all. This is easy stuff, easy enough for young children. Children can sing in parts, although they cannot sing the bass and tenor range. Once the boys’ voices have changed ,they can sing the tenor and bass parts.
And–I know I’m going to get blasted for this, but…PROTESTANTS have been singing in 4 parts for eons! If they can do it, surely Catholics can do it, too.
It’s really, really easy stuff.
The reason you need the parts written out is that most people do not have the parts memorized, and they are not always instinctive. I know a lot of the alto parts to traditional hymns because I have sung the alto since I was in grade school. And some of the parts I can make up as I go along because I play piano and I can “feel” where the part should be in my voice.
But some of the hymns and songs are hard to improvise parts for, and people like me ending up sounding really off, which is disturbing to others.
Here is a link to an example of a typical Protestant hymn out of a hymnal (I love this hymn, BTW):
cbcg.org/Sheet_Music/32.pdf
You see, it’s not complicated at all. To those who read music, this is as easy to read as a children’s book.