Bootstrap CSS class .modal-fullscreen-md-down

    <button type="button" class="btn btn-primary" data-bs-toggle="modal" data-bs-target="#exampleModal">
    Launch demo modal
    </button>

    <div class="modal fade" id="exampleModal" tabindex="-1" aria-labelledby="exampleModalLabel" aria-hidden="true">
        <div class="modal-dialog modal-fullscreen-md-down">
            <div class="modal-content">
                <div class="modal-header">
                    <h5 class="modal-title" id="exampleModalLabel">Modal title</h5>
                    <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
                </div>
                <div class="modal-body">
                    Lorem ipsum dolor sit amet consectetur adipisicing elit. Fugiat, velit.
                </div>
                <div class="modal-footer">
                    <button type="button" class="btn btn-primary">Read more</button>
                    <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
                </div>
            </div>
        </div>
    </div>