A:Answer Yes, all the current 27" iMacs have a small door in the back, operated by pushing in the button. Inside are 4 slots for additional RAM, which you can put in yourself.
The 21.5" iMacs are the ones without a door, although there are difficult procedures on the web about how to open it up; latest 21.5" imacs [as of March, 2018], have replaceable RAM, but it's hard to get to, you have to open the monitor and remove lots of cables and parts to get to the memory slots and that voids any Apple warranty.
HTH,
Sal