UPC vs FNSKU barcodes

First time Amazon seller here. I’m having some difficulty understanding some details of the process. Here are some questions for whoever feels charitable enough to help me out.

1: does the FNSKU need to be on the outside of the package And/Or on all the individual units?

2: can I use a UPC instead of an FNSKU on individual units?

3: does the parcel that I’m shipping to Amazon need all 3 barcodes (UPC FNSKU and carrier code)

4: what is the difference between FNSKU and SKU codes

5: do I need to apply these barcodes myself?