How to modify a URL in a cypress test?

Question

I have a Cypress test that I wrote, and to avoid writing a second test, I'd like to be able to take the resulting URL, and do two things with it:

1) I'd like to modify it to force it to another page

2) While doing #1, I'll also need to extract a dynamic value from the URL to be used in the modified URL.

I have cy.url() but I don't know how to capture the value I need and set it as a variable. Does anyone know of a way to do this? I'm a new coder and still learning javascript so this might be easy and I just don't know how to go about it. Any advice is greatly appreciated!