Yes, people can build based on the REN Protocol - own DApps (DEX is one of the DApps options)


The main difference from 0x is that REN brings compatibility with other blockchains for Daps built on its basis (also completely different tokenomics).
For example, you can get BTC liquidity on Uniswap, when Uniswap connect to REN.
Also, REN gives an opportunity to carry out private transactions.


You ask questions that can be answered for a very long time, so I tried to give the shortest answers.
If you have any more questions, feel free to ask.
More information:
https://renproject.zendesk.com/hc/en-us/categories/360000817693-Community-Resources
https://blockonomi.com/ren-protocol-guide/
https://medium.com/l33t/the-ren-project-ren-a-brief-overview-2019-e9dc0a5a6a98