.overlay on an element and wrap the element's primary content with .overlay-wrapper and the overlay content with .overlay-layer 
        and set the overlay backgroud color with opacity with .bg-white .bg-opacity-5:
    <div class="row">
    <div class="col-lg-4">
        <!--begin::Card-->
        <div class="card  overlay overflow-hidden">
            <div class="card-body p-0">
                <div class="overlay-wrapper">
                    <img src="assets/media/stock/600x400/img-1.jpg" alt="" class="w-100 rounded"/>
                </div>
                <div class="overlay-layer bg-dark bg-opacity-25">
                    <a href="#" class="btn btn-primary btn-shadow">Explore</a>
                    <a href="#" class="btn btn-light-primary btn-shadow ms-2">Purchase</a>
                </div>
            </div>
        </div>
        <!--end::Card-->
    </div>
    <div class="col-lg-4">
        <!--begin::Card-->
        <div class="card  overlay overflow-hidden">
            <div class="card-body p-0">
                <div class="overlay-wrapper">
                    <img src="assets/media/stock/600x400/img-2.jpg" alt="" class="w-100 rounded"/>
                </div>
                <div class="overlay-layer bg-dark bg-opacity-25 align-items-end justify-content-center">
                    <div class="d-flex flex-grow-1 flex-center  py-5">
                        <a href="#" class="btn btn-primary btn-shadow">Explore</a>
                        <a href="#" class="btn btn-light-primary btn-shadow ms-2">Purchase</a>
                    </div>
                </div>
            </div>
        </div>
        <!--end::Card-->
    </div>
    <div class="col-lg-4">
        <!--begin::Card-->
        <div class="card  overlay overflow-hidden">
            <div class="card-body p-0">
                <div class="overlay-wrapper">
                    <img src="assets/media/stock/600x400/img-3.jpg" alt="" class="w-100 rounded"/>
                </div>
                <div class="overlay-layer bg-dark bg-opacity-25 align-items-start justify-content-center">
                    <div class="d-flex flex-grow-1 flex-center py-5">
                        <a href="#" class="btn btn-primary btn-shadow">Explore</a>
                        <a href="#" class="btn btn-light-primary btn-shadow ms-2">Purchase</a>
                    </div>
                </div>
            </div>
        </div>
        <!--end::Card-->
    </div>
</div>.overlay-block on an element to display its overlay content by default:
    <div class="row">
    <div class="col-lg-4">
        <!--begin::Card-->
        <div class="card overlay overlay-block">
            <div class="card-body p-0">
                <div class="overlay-wrapper">
                    <img src="assets/media/stock/600x400/img-1.jpg" alt="" class="w-100 card-rounded"/>
                </div>
                <div class="overlay-layer card-rounded bg-dark bg-opacity-25">
                    <a href="#" class="btn btn-primary btn-shadow">Explore</a>
                    <a href="#" class="btn btn-light-primary btn-shadow ms-2">Purchase</a>
                </div>
            </div>
        </div>
        <!--end::Card-->
    </div>
    <div class="col-lg-4">
        <!--begin::Card-->
        <div class="card  overlay overflow-hidden">
            <div class="card-body p-0">
                <div class="overlay-wrapper">
                    <img src="assets/media/stock/600x400/img-2.jpg" alt="" class="w-100 card-rounded"/>
                </div>
                <div class="overlay-layer card-rounded bg-dark bg-opacity-25 align-items-end justify-content-center">
                    <div class="d-flex flex-grow-1 flex-center  py-5">
                        <a href="#" class="btn btn-primary btn-shadow">Explore</a>
                        <a href="#" class="btn btn-light-primary btn-shadow ms-2">Purchase</a>
                    </div>
                </div>
            </div>
        </div>
        <!--end::Card-->
    </div>
    <div class="col-lg-4">
        <!--begin::Card-->
        <div class="card  overlay overflow-hidden">
            <div class="card-body p-0">
                <div class="overlay-wrapper">
                    <img src="assets/media/stock/600x400/img-3.jpg" alt="" class="w-100 card-rounded"/>
                </div>
                <div class="overlay-layer card-rounded bg-dark bg-opacity-25 align-items-start justify-content-center">
                    <div class="d-flex flex-grow-1 flex-center py-5">
                        <a href="#" class="btn btn-primary btn-shadow">Explore</a>
                        <a href="#" class="btn btn-light-primary btn-shadow ms-2">Purchase</a>
                    </div>
                </div>
            </div>
        </div>
        <!--end::Card-->
    </div>
</div>.overlay-block class for blocking an element content.
    <div class="card shadow">
    <div class="card-header">
        <h3 class="card-title">Content Blocking</h3>
    </div>
    <div class="card-body">
        <div class="overlay overlay-block card-rounded">
            <div class="overlay-wrapper p-5">
                Lorem Ipsum is simply dummy text...
            </div>
            <div class="overlay-layer card-rounded bg-dark bg-opacity-5">
                <div class="spinner-border text-primary" role="status">
                    <span class="visually-hidden">Loading...</span>
                </div>
            </div>
        </div>
    </div>
</div>.overlay-block class for blocking card element.
    <div class="card card-border overlay overlay-block">
    <div class="card-header">
        <h3 class="card-title">Card Blocking</h3>
    </div>
    <div class="card-body">
        <div class="overlay-wrapper p-5">
            Lorem Ipsum is simply dummy text....
        </div>
        <div class="overlay-layer card-rounded bg-dark bg-opacity-5">
            <div class="spinner-border text-primary" role="status">
                <span class="visually-hidden">Loading...</span>
            </div>
        </div>
    </div>
</div>.overlay-block class block a modal's content.
    <button type="button" class="btn btn-primary me-2" data-bs-toggle="modal" data-bs-target="#exampleModal">Launch modal</button>
<!--begin::Modal Body Overlay-->
<div class="modal fade" id="exampleModal" tabindex="-1" role="dialog" aria-labelledby="exampleModalLabel" aria-hidden="true">
    <div class="modal-dialog modal-dialog-centered" role="document">
        <div class="modal-content modal-content">
            <div class="modal-header">
                <h5 class="modal-title" id="exampleModalLabel">Modal Title</h5>
                <!--begin::Close-->
                <div class="btn btn-icon btn-sm btn-active-light-primary ms-2" data-bs-dismiss="modal" aria-label="Close">
                    <i class="ki-duotone ki-cross fs-2x"><span class="path1"></span><span class="path2"></span></i>
                </div>
                <!--end::Close-->
            </div>
            <!--begin::Overlay-->
            <div class="modal-body overlay overlay-block">
                Lorem Ipsum is simply dummy text ...
                <!--begin::Overlay Layer-->
                <div class="overlay-layer bg-dark bg-opacity-5">
                    <div class="spinner-border text-primary" role="status">
                        <span class="visually-hidden">Loading...</span>
                    </div>
                </div>
                <!--end::Overlay Layer-->
            </div>
            <!--end::Overlay-->
            <div class="modal-footer">
                <button type="button" class="btn btn-light-primary fw-semibold" data-bs-dismiss="modal">Close</button>
                <button type="button" class="btn btn-primary ">Ok</button>
            </div>
        </div>
    </div>
</div>
<!--end::Modal Body Overlay-->