That's a really good question. And what @Kelheor pointed out was really interesting; that becoming a featured original may actually earn some creators less money than doing really well on CANVAS. Likely because you need to work so much faster, and to do that, you need to pay assistants. (It certainly helps give some insight into why a creator may turn down the offer to become an original.)
To answer your question though, they pay the assistants out of their wage. I think WEBTOON may contribute a bit toward it, but it depends on the individual creator's contract. I'm not sure about Tapas.
Personally, I'd rather go on pause and release one shorter season per year, and pause for the remainder, than work year-round with assistants, if it was going to work out to be the same money. And it seems some featured creators do that. But I suppose it depends on the contract they have, and how much story they have to tell.