Add “page-a” slug to category link if user visit category link from “page-a”

You would want to filter the category link to add the custom variable to the category link. You can make use of the get_term_link filter to filter the category link address

Here is a very basic idea:

Suppose that we, taken from OP, that you need to add the custom variable only to terms belonging to the build in taxonomy category, and only when we are on a real page and we want to add the page ID as value to the URL, we can create a filter function to just do exactly that, and by using add_query_arg(), we can add a new query variable, lets call it frompage, to the URL.

Here is a basic function, which you can extend and modify as you see fit. I have commented the code to make it easy to follow

add_filter( 'term_link', function ( $termlink, $term, $taxonomy )
{
    // If this is not a page, return the unmodified term link
    if ( !is_page() ) // Change this to what is specific to your needs
        return $termlink;

    // Only target the build in taxonomy 'category'
    if ( $taxonomy !== 'category' ) // Adjust to your exact taxonomy
        return $termlink;

    // Get the current viewed paged id
    $current_id = get_queried_object_id();

    // Make sure we actually have a value, if not, return the term link
    if ( $current_id == 0 )
        return $termlink;

    // If we reached this point, we are good to go and should add our query var to the URL
    $termlink = esc_url( add_query_arg( ['frompage' => $current_id], $termlink ) );

    return  $termlink;
}, 10, 3 );

You can then retrieve the value as follow:

$value = filter_input( INPUT_GET, 'frompage', FILTER_VALIDATE_INT ); 
if ( $value )
    echo $value;

Note that I have used filter_input here to get the $_GET value. That is because that filter_input validates if the passed value exists and either returns it or returns false. Also, you can pass a filter to be used on the value, in this case FILTER_VALIDATE_INT. All in all, it is a much better and safer way than $_GET( 'frompage )`