Boostrap why different image sizes?

Question

Images

Uploaded pictures of different sizes. Why doesn't the size automatically adjust to the block size? And that all cards were the same size.

THE CODE

<div class="row">

        <!-- Column for card-->
        <div class="col-lg-4 mb-2">
            <div class="card mx-auto" style="width:18rem;">
                <img class="card-img-top" src="https://www.istores.sk/iphone-11-64gb-black_ig108246.jpg?attname=thumbnail_490&attpedid=52" alt="Card image cap">

                <div class="card-body">
                    <h5 class="card-title">
                        Geeks for Geeks
                    </h5>
                    <p class="card-text">
                        to improve your computer science
                        knowledge.
                    </p>


                    <a href="#" class="btn btn-success">
                        Click me
                    </a>
                </div>
            </div>
        </div>

        <!-- Another column for card -->
        <div class="col-lg-4 mb-2">
            <div class="card mx-auto" style="width:18rem;">
                <img class="card-img-top" src="https://cdn.modernaskola.sk/946_15/9/mobilny-telefon-apple-iphone-se-9dn.jpg" alt="Card image cap">

                <div class="card-body">
                    <h5 class="card-title">
                        Geeks for Geeks
                    </h5>
                    <p class="card-text">
                        to improve your computer science
                        knowledge.
                    </p>
                    <a href="#" class="btn btn-success">
                        Click me
                    </a>
                </div>
            </div>
        </div>

        <!-- Another column for card -->
        <div class="col-lg-4 mb-2">
            <div class="card mx-auto" style="width:18rem;">
                <img class="card-img-top" src="https://s.s-bol.com/imgbase0/imagebase3/extralarge/FC/6/2/9/8/9200000105738926.jpg" alt="Card image cap">

                <div class="card-body">
                    <h5 class="card-title">
                        Geeks for Geeks
                    </h5>
                    <p class="card-text">
                        to improve your computer science
                        knowledge.
                    </p>
                    <a href="#" class="btn btn-success">
                        Click me
                    </a>
                </div>
            </div>
        </div>
    </div>