Why is the page code (Ctrl + U) different from the browser client code?

Question

JavaScript

There is a question regarding the implementation of a site with dynamic content, i.e. when changing the url, for example
.../?lang=javascript/New%20York, the word javascript and New York is taken and inserted in the right place on the page, I implemented this using js, for example like this:

//Title Page
    var job = loc_url.split('/')[0];
    //Description
    var location_job = loc_url.split('/')[1];
    //Someone word
    var word = job;
        //url dont have location
        document.title = 'Remote ' + job + ' professionals available now';
        $('meta[name="description"]').attr('content', 'Quality remote ' + job + ' talent available to work with your in-house team within 4-6 weeks. Access hand-picked specialists from Europe’s largest and most affordable software engineering hub. Scale your ' + job + ' capabilities while staying lean and agile');
        $('header .content .text-block h1').html('Paying too Much for ' + job + ' software developers?');
        $('header .content .text-block h2').html('Find out the real market rates for quality remote ' + job + ' software developers and always pay the right price');
        $('.spanScroll').html('(scroll to request resumes from our pool of available ' + job + ' engineers)');
        $('.tap-our h2').html('Tap our on-demand developers to seamlessly integrate with your in-house team!');
        $('.third-block h2').html('we have a number of excellent ' + job + ' candidates in our database. Join the ranks of our happy clients who have augmented their ' + job + ' teams with remote developers sourced by us');
        $('footer p').eq(1).html('to get resumes of ' + job + ' software developers available to work for you right now');

So this is the essence of my question, how to make this js code work when viewing the page code:
view-source: ...?lang=javascript/New%20York, i.e. now if you press Ctrl + U and look at the code, then the fields are empty, i.e. js code is not working.
As far as I understand it, this is very important for SEO.

Answer 1

Don't do it at all. If you need something to be in the source code of the page when it is received by the browser, generate it on the server.