You need to heat the grain in a closed pressure vessel, wait for the pressure to build up and then suddenly release it. There are Chinese popcorn machines that can do that:
with rice at least sadly no. though you can puff it using hot salt or sand, as is popular in India
most grains lack the hard watertight shell popcorn has that allows pressure to build up in the kernel
things lacking the hard shell usually have to be puffed via a puffing gun
look up puffing guns and popcorn cannons for a fun internet info hole 👍
Doing it with rice is not super hard. It needs to be cooked the regular way first, so if you have leftovers you can try with that. Just dry it out, maybe in an oven on low for a little bit, then give it a dunk in hot oil, around °325-°350. It puffs right up, only takes a couple seconds.
You can puff rice noodles the same way, but you don't have to cook it first since it's already been processed. I like to serve Mongolian beef or some other kind of stir fry over it.
I think there's a pretty solid evidential record that cooking foods in the microwave until they explode generally makes them worse and/or makes a fucking mess.
In Korea, there are these wandering food exploders. You go to them with your own peas, beans, anything dried really and they put them in a heated pressure chamber. After heating, the pressure is rapidly released, which causes the small amount of water remaining in the food to boil off and turns them into a fluffy matrix similar to rice crispies.
The machine makes a sound very similar to a gunshot, so the operatora shout something to let people know.
Rice crispies and some chips are made with a similar process. This also works for dough.