Bootstrap 4 no longer has input-group-btn #8

In the tutorial & even after your .NET Core 3 update, the span tag in List.cshtml uses the bootstrap tag input-group-btn <span class="input-group-btn">

Per the documentation from Bootstrap:

Input groups

Input group addons are now specific to their placement relative to an input. We’ve dropped .input-group-addon and .input-group-btn for two new classes, .input-group-prepend and .input-group-append. You must explicitly use an append or a prepend now, simplifying much of our CSS. Within an append or prepend, place your buttons as they would exist anywhere else, but wrap text in .input-group-text.

Validation styles are now supported, as are multiple inputs (though you can only validate one input per group).

Sizing classes must be on the parent .input-group and not the individual form elements.