From the exterior shots, Mary’s apartment (https://www.google.com/search?q=Mary+Richards%27+apartment&tbm=isch&ved=2ahUKEwjrhPqvxYHuAhUHDq0KHVdsD58Q2-cCegQIABAA&oq=Mary+Richards%27+apartment&gs_lcp=CgNpbWcQAzIECAAQHjIECAAQHjIGCAAQCBAeOgcIIxDqAhAnOgQIIxAnOgIIADoFCAAQsQM6CAgAELEDEIMBOgQIABBDOgcIABCxAxBDOgoIABCxAxCDARBDULOnAljE8gJg4vMCaAZwAHgBgAH1AYgBkxySAQYwLjI2LjGYAQCgAQGqAQtnd3Mtd2l6LWltZ7ABCsABAQ&sclient=img&ei=FafyX6umGYectAXX2L34CQ&bih=757&biw=1600) seems to be the finished attic.

https://i.pinimg.com/originals/15/7e/6a/157e6ab9790bcfbb2938f455af2f9a19.jpg

Phyllis on Floor 1

Mary on Floor 2

Rhoda on Floor 3

not sure about the outside camera shots you saw causing the confusion

lots of inside shots of Mary ... on her rear floor 2 outside deck