Are you thinking of angels and harps and fluffy white clouds?
Left to her own devices, Young Ju would probably be imagining all of those things too, but Young Ju has her parents and Halmoni talking non-stop about "Mi Gook" (or America) and how amazing the place is.
So it kind of makes sense that Young Ju would think of Mi Gook/America as heaven since it's a place where "everyone can make lots of money," where "all the uhmmas… are pretty like dolls" and "live in big houses" (4.2). In other words—it's easy street. And for a kid who isn't thinking in symbols and metaphors, heaven and America are real places where good people can finally reap the material rewards of all their hard work.
And that's because it's other people—a.k.a. the adults in the family—who treat America like a metaphor, like it's heaven. It's their dreams of what America might mean that influence the way Young Ju imagines heaven, and therefore America. The very words "Mi Gook" can also, by the way, make all the "mean eyes over the rice bowl" disappear; Apa even "smiles so big [Young Ju] think[s] maybe he is a doggy like Mi Shi" (4.3). And what kid doesn't like it when the mean eyes of adults disappear?
No wonder Young Ju thinks Mi Gook is magical and heavenly; the mere mention of it makes her family seem happy. We don't think we need to tell you though, that this metaphor gets squashed pretty quickly in the book since America clearly isn't heaven (which Young Ju figures out the moment she arrives).