What’s the difference?!

So I was pre-making my sons formulas, leaving it in the fridge and just heating it up. My mom told me that it isn’t good because heating it up like that takes all the nutrients from it; so I stopped doing it; I just started heating up the water and adding the formula after.

Now what I don’t get is there is pre made formula in the jugs you just heat up so I don’t get what the difference is between me heating up the formula I make compared to the pre made formula when it comes to heating it up?

I’ve tried just giving it to him cold he won’t take it like that. He’s actually tried but spit it all back up.