@Carnelianstar (sorry i thought it was a statment not a question)
onomatopeia is like comic books "WHAM" "POW" "WAZAM!!"
TLDR:
and a subspace it basically a big space, so like in reincarnation manhwas/animes, or reborn into a video game ones, the mc has a storage/inventory that has unlimited space. in this case, since soo-yeon doesn't have any special abilities herself, she cant summon a subspace because her subspace is the case itself. But it follows her will (but it cant read her mind, she has to announce it. another quirk of hers, announcing every move in battle) this is for reasons to be revealed later. Her sub space, since the case follows her will her subspace takes the form of a beach, her safe space.
so to sum it up a subspace is a unlimited inventory. a space with no concept of time or space, so if she put a grilled cheese deluxe in there it wouldn't get old.