They played behind almost everyone.
Yep.
Mamas & Papas, Sonny & Cher, the Monkees, the Beach Boys, Simon & Garfunkle, the Association, just about every TV Commercial jingle.
Wiki updated their page recently with a "selected recordings" list, don't remember seeing that before:
https://en.wikipedia.org/wiki/The_Wrecking_Crew_(music)
I read in Crosby's autobio a while back that there were a lot of concerns re musicianship and competency early on when the band was first getting together. to go from there to John Lennon's fave band in 2-3 years is really impressive.
It was covered in that article that they were still pretty ragged when they toured England in '65.
I think the real reason Lennon loved 'em so much was their role as ambassadors of the new, uh, "California mind-set".
John and George took LSD for the second time (Ringo for the first time)when they were in LA in August '65 (Paul was still holding out):
https://www.beatlesbible.com/1965/08/24/lsd-los-angeles-byrds-peter-fonda/
Extracted from that link:
"Among the visitors on this day were Eleanor Bron, whom had appeared with The Beatles in Help!,
Roger McGuinn and David Crosby of The Byrds, and Daily Mirror newspaper journalist Don Short.
And:
"There were girls at the gates, police guards. We went in and David, John Lennon, George Harrison and I took LSD to help get to know each other better. There was a large bathroom in the house and we were all sitting on the edge of a shower passing around a guitar, taking turns to play our favourite songs. John and I agreed Be-Bop-A-Lula was our favourite '50s rock record.
I showed George Harrison some Ravi Shankar sounds, which I'd heard because we shared the same record company, on the guitar. I told him about Ravi Shankar and he said he had never heard Indian music before.
You can hear what I played him from The Byrds' song Why. I had learned to play it on the guitar from listening to records of Ravi Shankar.
-Roger McGuinn"
That was also the day that Peter Fonda's rambling inspired John to write "She Said She Said"
I think that day was pivotal in the direction they took on
Revolver, indeed, in their history as a band, given the significance of
Revolver heralding things never before heard in the world of pop music..