so we have our catalog page and the header page. we direct the user to a diff catagory in our catalog page based on the variable sent to the server.
my question is this, if the variable is /?cat="books" the user will be directed to the CATALOG page am I right? our catalog page is dynamic, the content inside him changed based on the variable sent to the server?
Yes. This is stated in the video's description:
Our catalog page needs to display different categories depending on what the user selects
And you can see it in the address bar when previewing the selections: