I know in some parts of different country's they let the pups go early, but 7 weeks is very young...is that the norm in Korea? I'm Korean btw.
Make sure she's eating well & often & good thing that she's fenced off. It will make potty training easier & she won't spend all her energy & calories by roaming around thus making her crash & get hypoglycemic.
About the stairs, we live in a condo which is just like an apt w/steep stairs & my guys do fine, they fly up & down the stairs. I would gate off the stairs so she doesn't have access to it. If I'm upstairs, they all follow me & I gate off the access to go down & vise versa. I also have their pee areas in the bathrooms (upstairs & down) since we don't have a yard. GL w/your new pup & looking forward to more pics & fun stories!