stupid question please don't judge!

why is formula milk-based or soy-based? How is it not the same as giving them milk before 1 year, which isnt recommended? So my baby and I have been ebf for 4 months but as of recently I noticed my supply has diminished, especially at night. well anyways, I've been thinking about supplementing with formula since my milk stash is close to gone. when I was shopping I noticed it said milk based, I'm a FTM and have no understanding of this especially since I wasn't expecting to use any formula I never asked about it.