The screenshots taken on an simulator are stored usually on desktop of the system that you are using.
There are multiple scenarios in which screenshots could have been taken, some of them are mentioned below.
When the images are taken using "Command" + S, or from File menu's New Screenshot option, They are usually stored by name similar to "Simulator Screen Shot - iPhone 7 Plus - 2018-12-26 at 18.18.14" which consists of Simulator running currently followed by date in YYYY-MM-DD at HH:MM:SS format.
If they are taken with mac's "Command + shift +3" or "command + shift + 4" buttons, they are saved on the desktop but with different name which is similar to "Screen Shot 2018-12-24 at 8.35.20 PM" which just says "Screen Shot YYYY-MM-DD at HH:MM:SS".
If you take a screenshot from within your app running on the simulator, and then save it to photos app on that simulator, then it is stored in some library folder of that specific Simulator.
The address is usually