Converting vs. Spot - What is the difference?

Hello. Can you explain the difference between these? I know that you can't convert all of the coins but let's say that I have BUSD and I want to buy BTC. What is the difference between buying BTC, using my BUSD, from spot-market price / converting my BUSD to BTC in trades-convert?

Is there a fee or a buying price difference? I use convert because it gives me a preview of how much of a coin I will get in return, but when in spot, it doesn't.

Let's say I have 100 dollars in BUSD and I want to buy BTC. What would be the fee in;

1- Spot-market price

2- Trades-convert price

Thanks.