I often will say that training a muscle group more than 1X week is more beneficial for muscle growth than only training it 1X week.
However, there are now studies that take new variables into account that prior studies did not, and it now shows no effect of training frequency on muscle growth. With a set amount of volume (think sets, reps and load per muscle group a week), it doesn't matter if you train a muscle once, twice or more times per week.
BUT! There is a caveat.
As muscle volume gets higher, it warrants more frequency than training a muscle group just one day a week because you can't get it all in one day.
At the beginning, our muscles don't require as much volume but as we train longer and get stronger, we require more volume to stimulate growth.
Let's think about getting 25 sets of exercises ONLY for our quads in a week. This would obviously be more achievable over multiple training days. There's no way we would be able to train with adequate effort in every set if we tried to smash this all in one day.
So, the take-home here is that there isn't necessarily anything "magic" about training a muscle more than 1X week other than it is what allows us to get adequate volume per muscle group.
I hope this is useful, and let me know if you have any follow up questions or comments!
Study: https://pubmed.ncbi.nlm.nih.gov/30558493/