Does Canvas detect copy and paste?
Partly
Not in a normal assignment submission. Inside a locked Canvas quiz with proctoring enabled, paste and tab events can be logged.
How it works
Standard assignment upload records the file and a timestamp. Quiz log data and third-party proctoring extensions are what capture in-page behaviour.
The part people miss
What is logged depends entirely on which tools your institution enabled. There is no single answer across schools.
Flagged for something you wrote yourself?
There is a right order to handle it in, and editing the document first destroys your best evidence.