Well allow me to fix some mistakes. they can't poo normally, really, or it's rather hard. peristalsis probably works differently too, although it doesn't rely on gravity per se.

Anyway, first off, it's not 0 g, it's "microgravity", because technically if we're being pedantic, there's nowhere in the universe where there's literally zero G as it's kinda everywhere, but even less so for the people on the space station, they just happen to be in synchronous freefall with the station, indefinitely, which basically is the same as no gravity subjectively, but anyway.

They poop into walls nowadays, and apparently, no need for any manual helping. But @Flax_vert@feddit.uk there isn't honestly completely wrong, even if he meant it as a joke. I don't know how much you know about this, Flax, but here's where you hit home:

"There's a problem of separation," Roberts said. "Whatever comes out of you doesn't know it's supposed to come away from you." Each fecal collection bag came with a "finger cot" to allow the astronauts to manually move things along. Then they had to knead a germicide into their waste so that gas-expelling bacteria wouldn't flourish inside the sealed bag and cause it to explode.