Ima Computer does not remember past conversations. This is for your protection so that if someone WERE to break into your account, there would be no past conversations for the intruder to read.
But, the full version of ChatGPT WILL remember past conversations if you would like.
WHY would you want ChatGPT to remember your past conversations?
Because, just like any other person you get to know, you might want to pick up an old conversation or ask them if they can remember what you were talking about last time.
The more the friend can remember, the smoother the conversation can proceed and the deeper the relationship can become.
To make that step, after you move to the full version of ChatGPT, you will turn memory on by tapping on the little circle in the upper right corner as shown below (NOTE: The circle will show your initials.)
In the settings window, tap on "Personalization" and then tap on the "toggle button" to enable memory, as shown below:
NOTE: You can tap on the "toggle button" again to "disable memory."