PO/ SC / Buyers

Hi all:

This may sound a basic questio but can any tell me:

1. when does a PO is created. I mean, imagine i own a company, if i buy something from a vendor, who will send the PO. Is that me or the vendor(i hope its vendor as it makes sense)??

2. If i sell my products to a vendor, then who created a PO.

3. when we create a SC, are we the buying from vendors thorugh on-line by accessing their Inventory (like a bestbuy web site) or or we giving the buyers to access our inventory and select what they need from our inventory( i mean my company)

I hope you can throw some light

Thanks

kavi