> Ctrl + C to cancel a command in terminal but Cmd + C to copy text in terminal. Ok, then why Cmd + T to open a new tab in terminal instead of ctrl?

I don’t see what’s inconsistent there. Cmd+key sends a command to your terminal application, Ctrl+key sends a command to a process running in your terminal.

That’s why, for example, Cmd+C copies selected text in your terminal window, while Ctrl-Y yanks text in many applications (including many shells, via readline).