As the user @antonio mentioned in a comment the correct answer to the OP's original question is:
C-x (Your prefix
Ctrl+a in the case of the OP,
Ctrl+b by default)
& and confirm with
This is A single shortcut which kills the current session.
Not merely detaching from it as
If you run
tmux ls you'll see that with
d the session is still running in the background and that with
y it is not but that any other sessions running are still there. You can re-attach to these background sessions with
tmux a -t <session> where session is a name number.
This way there is no need to type a command at the prompt (accessed with
kill-server kills all sessions not merely the current one
kill-session is equivalent to
As @Vaisakh-k-m pointed out
w gives you an interface to select windows across multiple sessions which you can select with the arrow keys and
t to toggle the tagged status then
X to kill the tagged windows and
y to confirm this action.