How do I override the terminal command "rm" to perform the "rm -i" command by default?
Use an Alias.

The alias command allows the user to launch any command or group of
commands (including options and filenames) by entering a single word

alias rm=&quot;rm -i&quot;

The next time you issue the command rm it will issue rm -i
You can also make a new alias in case you want to keep rm functioning as it does by default.  To create a new alias
alias rmi=&quot;rm -i&quot;

Now, rm will have it's default behavior and rmi will have the -i option
To make it permanent, just add it to your ~/.bash_profile.  This way, everytime you open a new shell, your alias will be ready to go.