I'll try to answer some of your questions the way I understand it. Bitshares DEX (bitshares.org) is the core wallet and exchange of all assets native to bitshares (meaning the assets made within bitshares).

Openledger is one of the few businesses built on top of Bitshares' core software. It's like a white label or private label in a way. Anybody can do their own exchange using Bitshares, actually there's quite a few right now like Rudex, Cryptobridge, etc.

They all differ from the core in that they can act as an escrow or broker for us to deposit and withdraw assets not native to bitshares. Assets like bitcoin, ether, steem, etc.

Thus you might see a prefix on these assets like OPEN.BTC, OPEN. ETH, etc for all assets guaranteed by Openledger. The same goes with other exchanges built on top of Bitshares (RUDEX. BTC, BRIDGE. BTC, etc)

It might seem quite complex to people upon seeing something like this, but improvements are on the way to make it look less confusing and easy to use.

With regards to the 'extraction fee', I'm not so keen on how big it is compared to other exchanges but I've been trading on other exchanges and I think it's not that high. You can also get as much as 80% back on the fees once you become a lifetime member which costs approximately less than $160 right now.

Hope I answered some of your questions.