Understanding Transaction Time in Sentry - React.js

What are the components of browserTracing transactions? From the dashboard and Docs I can see that HTML, CSS-JS-Images fetch are shown as operational breakdown. Breadcrumbs show the XHR calls.

So does the transaction time include all XHR calls as well? What if some XHR call gets success after sentry sends the data? What if some XHR calls get non 200 first but then succeed? All these will be counted in total transaction time?

Also, for each trasactionContext, I have a name and there are multiple transactions under it, with unique IDs given by sentry. Each transaction ID shows the breakdown. Can I somehow aggregate the transaction breakdown? For example have a CSS loadtime for all the transaction IDs with same name?

Thank you for your help.