1) The notes are printed by the bank of Canada located in Ottawa Ontario.
2)There are many many (hundreds) of letter combinations in the 1's and 2's. I think somewhere on this site you might be able to find a list of all prefix letters.
3)There are some that are worth more than others, particully those with less printed for example prefix V/F is worth slightly more.
The others that are worth a lot more than many collectors search for are those prefix with a * in front
*A/A *D/O *I/O *S/O etc
These are worth much more than other prefix with no * and may be worth hundreds of dollars
*C/I and *V/V are the most expensive with fewer than 10 known each and can go up to as high as $15,000 in price