Question: SBD conversion to STEEM on steemit

Hi guys,

I am studying STEEM these days as a way to study blockchain and I'm stuck with two issues now.

How conversion ratio btw SBD and STEEM is determined on Steemit?(lets forget about outside exchanges)

I thought SBD can be pegged to US$1 by interest rate and conversion rate controlled by witnesses.

So my question is

1. What is price feed? Are the witnesses gathering price information of 'STEEM' from exchanges? If so, how STEEM : SBD conversion rate is determined?
   or, is that price of 'SBD' that witnesses are proposing, and we call that price feed?

2. Are people using SBD → STEEM conversion on Steemit? it seems that it would be much better to sell SBD on exchange and purchase STEEM on the exchange(if Steemit sticks to 1SBD = $1 as Steemit intends to maintain the peg)

Thanks for reading. God bless you