The Instant Buy/ Sell looks at what's feasibly available on the market, it doesn't always guarantee the best rate.
If you'd prefer to make more advanced trades where you can specify the price and place limit orders I'd recommend using our Exchange. I've linked in a guide on how to use this below.
Before the order is made, just before you enter your PIN, a confirmation is made for the buy/ sell.
In the background it gets fairly complex.
The Instant Buy/ Sell looks at the available liquidity and what's available at the value you're requesting it. The page then generates a quote for 60 seconds. If the order is executed, the order is still placed on the exchange, but in the background.
Any BTC order is made on the exchange, it's just easier and quicker to do it via Instant Buy and Sell.
Hope that makes sense