Perhaps interesting to note that in barbershop harmony, the roles are called Bass, Baritone, Lead, and Tenor -- and that's true regardless of the genders of the participants.
The "high voice wail" part that is sung by Brian and is featured on a lot of BV on Beach Boys records in the 60's is falsetto, but on lead vocals in the early years of the Beach Boys Brian was (in most cases) mainly attempting to replicate Bob Flanagan's part in the Four Freshmen, which was a jazzy barbershop tenor or even high tenor, not falsetto.
See, that's where I disagree. I think Brian was a sort of leggiero tenor type voice that mixed a lot of head voice into the timbre pretty low in his range and never needed to open his vocal folds to produce falsetto.
In his 'classic' 60s period, Brian would switch between normal (chest) voice and falsetto (head) voice all the time, whether doing backing or lead vocals. But at this time his 'normal' vocal range was so high that it could overlap with his falsetto range. By which I mean, some notes he could choose to since either chest voice or falsetto, depending on the dynamic of the song, A good example is Don't Worry Baby. By way of demonstration here's the opening sections, where I've highlighted
in yellow where (I believe) he switches to falsetto. But for sure there's very little difference in the timbre of his head and chest voice, which is unusual.
Well it's been building up inside of me
For oh
I don't know how longI don't know why
But I keep thinking
Some
thing's bound to go wrongBut
she looks in my eyesAnd
makes me realize
when she says Don't worry, baby
Everything will turn out alright