How to open a file from the same folder on the site in an animated window?

Question

css

I have a simple interactive site for a diploma,

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these are 3 buttons, after clicking on any it gives a window with an empty background, you need to put some other file that I have on it.

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it should turn out like this, but I just have an empty background, but how can I put the actual file on this background? Button code attached:

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<div class="portfolio" id="portfolio">
        <div class="container">
          <h2 class="section-header">Организации профсоюзов Республики Калмыкии</h2>
          <div class="row">
            <div class="col-md-4">
              <div class="portfolio-block">
              <img src="img/portfolio/1.jpg" alt="Работа №1">
              <a class="link cd-modal-trigger" data-work="work-1.html" href="#0">Официальный сайт</a>
              </div>
            </div>
            <div class="col-md-4">
              <div class="portfolio-block">
              <img src="img/portfolio/2.jpg" alt="Работа №2">
              <a class="link cd-modal-trigger" data-work="work-2" href="#0">ПРОФСОЮЗ РАБОТНИКОВ СВЯЗИ РК</a>
              </div>
            </div>
      
            <div class="col-md-4">
              <div class="portfolio-block">
              <img src="img/portfolio/3.jpg" alt="Работа №3">
              
              <a class="link cd-modal-trigger" data-work="work-3" href="#0">ПРОФСОЮЗ РАБОТНИКОВ ЗДРАВООХРАНЕНИЯ</a>
              </div>
            </div>
          </div>
        </div>
      </div>

In general, I have a photo as it is and how to do it, I just can’t understand, if I need to lay out the whole code in full