The protocol works as a primary market, which means that operating with the protocol the user creates (mints) and destroys (redeems) the tokens, therefore, as the DOC is a token with 1:1 parity with the usd, always its value is 1 usd. A DOC parity chart would be a perfectly horizontal line.
You can read the difference between primary and secondary markets in this blog post.

Yes. To issue a DOC, rBTC (bitcoin on Rootstock sidechain) is used and the collateral that makes the issuance possible also uses rBTC to issue BPRO.

You can use them in Krypton Market to buy through their Kripto Store or using the web extension. You can also save with performance in Tropykus.

When the price of bitcoin drops, if the issuance of DOC and BPRO remains constant, the Global Coverage falls. When the Global Coverage reaches the Target Coverage, if the bitcoin price continues to fall, the system activates mechanisms that encourage the provision of collateral to restore the balance.
If you want to know more about this topic you can read this blog article What if bitcoin had a flash crash?

In this scenario, if the amount of DOC remains constant the Global Coverage goes down. When it reaches the target level (currently at 4, that is, for each DOC there are 4 usd in rBTC of collateral) the protocol does not allow the issuance of new DOCs or the redemption of BPROs. This scenario produces another result: the leverage of the BPRO goes up, making it more tempting to provide collateral and rebalance the system.

This ratio is expressed by the Global Coverage value that you can find in the dApp under the Metrics section.