Yes you can define your own key bindings. The default is the same “prefix key” that ‘screen’ uses - Ctrl-b. Personally I like to use Ctrl-j because it’s easier to type and has fewer conflicts with shells and other things. Most of the key bindings that follow the prefix key seem natural to me, stuff like ‘c’ to create a new tab, ‘d’ to detach from tmux, ‘n’ to go to the next tab, etc., but all of that can be customized.
> The default is the same “prefix key” that ‘screen’ uses - Ctrl-b.
Isn't the default command key ctrl-A for screen? Which is why I've always had to change it (to ctrl-O which is much more sensible) because I can't live without ctrl-A in the terminal etc.
In screen, I just use a single backtick, as I very rarely use it for any other application in a terminal session (I prefer "$()" in shell, invariably edit TeX and Lisp in GUI Emacs, and the only only other thing that comes immediately to mind is opening the developer console in mpv and various video games).