If you mean that there is a subway running directly under the building you live in, then ‘there’s a subway below my place’ is fine. You could also say: ‘There’s a subway under my building’ ‘A subway runs under the place I live’ ‘The subway runs under the building I live in’

"There's a subway downstairs below my building" = you have to walk downstairs to ground floor exit level, and then there is the subway. Or there are also stairs to the subway when you get to street level of your building. this is a bit vague. "There are stairs down to the subway, below my building" "below my building there are stairs to the subway" I would use this sentence.

Hello :) You should say " the subway is down the building where I live"

Downstairs is fine, but it stands alone Usually it means in the same building but on a lower floor If your bedroom is on the second floor and a friend comes to visit Someone might say,"Your friend is downstairs."