What is the difference between a single apartment and a one-bedroom apartment?

Answer 1

Where I live, We have what is called "Bachelor" Apt, which I think may be the same as your "single Apt"

A bachelor apartment does not have a separate bedroom, the bed is put in a Jog (alcove) of the stting area.

So you have a bathroom, a small kitchen, small eating area and a bedroom/living room.

I bedroom Apt has a separate room for the bedroom.

Answer 2

They signify the same meaning.Both have similar amenities.
Depending on what part of the globe you reside,different
verbiage but same meaning.