I'd suggest you take your questions which aren't Tableau REST API specific to a website like Stackoverflow. You'll get more responses over there.
In this instance, you're asking how to parse certain elements from a block of XML. Here's an example on Stackoverflow - How can I access a single XML element's value using C#.net web-pages with WebMatrix? - Stack Overflow
Effectively you use XPath statements to 'query' the data and extract the element value.