I can do it in a few sentences. Non-bitshares assets like Bitcoin and Etherium cannot be traded on the bitshares blockchain because they are non-native tokens just like you can't send steem on the bitcoin blockchain. To solve this issue, gateways(open ledger, cryptobridge, lykee, and other potential gateways) deal with deposits and withdrawals. They accept bitcoin and issue a native asset on bitshares blockchain that represents bitcoin in the form of an IOU(open BTC, Bridge, btc). Gateways charge a fee for this deposit service and they charge a fee when you want your original bitcoin back. Every time their asset is traded on bitshares, they also receive a fee. These fees are much smaller than centralized exchanges. The actual trading of these native assets All happen on the bitshares blockchain. Bitshares charges a very small fee when users execute trades and when Open ledger and Cryptobridge create iou tokens(open litecoin) to represent non-native cyrptos. Hopefully, this helps.