Do the papers go through the same process when submitted through open access
Open access fee payment is after the paper is accepted. This is to prevent conflict of interest. It means peer review is conducted without knowing whether the paper is open access, and it means the editorial board / publisher is under no extra incentive to accept the paper because of the article processing charge.

In other words, if you submit your paper to Discrete Maths, you should make it all the way to the decision stage (i.e. an accept/reject decision) before you are asked whether you want to make your paper open access. If you say yes, then the production process changes - for example the publisher will use a different imprint on the first page that says you continue to hold the copyright. But that happens after the decision stage. Before it, everything remains the same.

As for why pay a large sum of money for open access: open access is not meant as a shortcut to skip peer review, it's meant as a way to make your paper publicly visible for free. If you publish something in a subscription journal, only those who subscribe to the journal can read it. If you publish something open access, anyone can read it. The core "idea" behind open access is that scientific research should be free to help progress humanity's knowledge as a whole. You can read more about it on the Wikipedia page.