does the plastic in the 2 liters break don the same as the plastic in the milk jugs?
It will break down, but over a much greater period of time.
how does plastic get into the food?
I don't think that plastic gets into the food. Rather, over time the plastic will off-gas various chemicals. There's much speculation as to precisely which chemicals, how much is off-gassed, how much is absorbed by the food, and whether or not the chemicals or the quantities absorbed into the food constitute a hazard. Name your plastic, and some Chicken Little will have an issue with it. I personally don't have any worries about storing consumables in PET, PETE, or polycarbonate. I would stay away from milk jugs and anything made of that same polyethylene as it is very gas permeable (ever notice that when you put that left over spaghetti in the fridge the milk takes on the taste of garlic?) If you're paranoid about the plastics, there's always glass.
what if I was to store rice in the milk jugs?
Then the rats and mice in your area would enjoy a nice feast.
A widely accepted procedure for grains is to put an empty mylar bag into a 5-gallon bucket, fill the bag with grain, displace the air inside with nitrogen or CO2, toss in a desiccant pouch or two, seal the bag, and seal the bucket.
Yeah that's how I've been doing it, but it would be nice to find a way that is cheaper and less time consuming.
IMO glass is to fragile for long term storage.