Mo was a phenom as a child, easily winning at national events which Cram and co had struggled at at similar ages, but he stagnated in his early 20's. He was not a part of a consistent group in those early years. He had his own coach, but he was nomadic. He was weak, couldn't sprint, but showed tremendous talent. I want to say it was Malmo who talked on here about Mo breaking 13 off minimal training (and possibly while sick, I forget?) in 2010 in Switzerland which was the point his then-coach started to believe he could be a world beater.
At the same time he was heavy. Before Farah went to Al Sal, he was competing at 58kg and had no muscle tone. Under Al Sal at age 27 came the bespoke strength programme, through which he went down to 52kg (lots of caveats from Fudge and Salazar out there about doing this carefully; you can make your own assumptions). But keep in mind, once again, that he was already a sub 13 guy, maybe a 12:50 guy on the right day, when he came to Salazar. And that was accomplished in an era when sub 13 was still something special, and he was doing it raw. Massive talent.
The weights and sprint emphasis in addition to the removal of 4-6 kg can't be understated. Salazar focused on marginal gains in ways other coaches did not at the time. He needed world beaters who could stay lean AND flat-out sprint. This meant real weight training. Look at Sifan's similar body change through the same tactics.
I want to stress how bad Farah's sprint speed was when he went to Salazar. There was an article I came across (or perhaps a video) way back in the day of AlSal explaining this uphill 200m sprint test he used to use for base setting for all of his new athletes. It translated directly to efficiency, strength, and finishing speed. Farah's was something like the worst he'd ever seen from a high-level runner, slower even than some of the females in the group.
I'll leave it there for now for brevity's sake. But you get the point. There is a logical explanation to the progression, and of course you're right to question if you can be that strong and fast naturally at that weight, but that's a topic for another day.
Hope this is helpful.