When you start a shell your history list is populated from your
.zsh_history file. Depending on options that you have set, when you close a shell you write your history list to that same history file. Until you close your or open new shells that history is self contained and not accessible from another shell.
There is a built-in zsh command to both write and read history from the
fc -W will write to the history file.
fc -R will read from the history file.