On YT when I lookup exercise or programming videos I always tend to go for 'science based' videos, from Jeremy Ethier, Jeff Nippard, Menno Henselmans etc

Especially Jeremy always comes up with charts of EMG Muscle activation. Goes like this: our target is quads, this exercise shows more Muscle activation than exercise B,C,D, E on the EMG, so it is most likely the best choice for building this muscle .

Is it true though? Is Muscle activation on the EMG a good indicator for muscle growth?

no, case in point look up the EMG of lats in a bench press then look at the average guys back:bench ratio at your gym. Or go bench then go do a lat movement or vice versa and tell me how much it impacts one of the other.

they're neat to look at and know but hold no real world weight when everyones proportions vary so greatly.

so for example in squats, some guys can squat and get massive quads yet their adductors will ****in suck, yet on EMG studies adductors are theoretically firing harder than the quads. Some guys will squat and their ass will grow like a weed but emg studies show the ass to be barely worth mentioning.

knowing the actual function of a muscle (its attachment points and the way the fibers run) is much more important than emg studies. And even that knowledge is pointless when all that matters at the end of the day is figuring out what works the best for you.