John Lennon believed that the important nuances about that relationship were lost in the public discourse, as an interview from 1975 shows.
George Martin was instrumental in transforming The Beatles into the global pop phenomenon they are known as today. For example, Rolling Stone wrote in 2016 that it was Martin who convinced Paul McCartney to "grab an acoustic guitar and just sing" what would become "Yesterday", when the band didn't know how to materialize the song that came to McCartney in a dream. He also introduced a string quartet to the band, Rolling Stone added.
In an interview with BBC's The Old Grey Whistle Test, John Lennon argued that neither The Beatles nor George Martin were solely responsible for the band's magic. Both parties had their fill of work and benefited from each other's input (transcribed by Ultimate Guitar):
"It's hard to describe the relationship. They either say 'Martin did everything,' or 'The Beatles did everything.' It was neither one. George had done little to no rock and roll when we met him, and we'd never been in the studio, so we did a lot of learning together."
"He had very great musical knowledge and a background, so he could translate for us and suggest a lot of things, which he did. He'd come up with amazing technical things like slowing down the piano and playing it slowly and putting it on and things like that, you know."
Source: Guitar World