I am following

_Layout.cshtml:

...
    <div class="container">
            <main role="main" class="pb-3">
                <a class="navbar-brand" asp-area="" asp-page="/Contacts/Index">ContactManager</a>
                @RenderBody()
            </main>
        </div>
...

Contacts/Index.cshtml:

@page
@model RazorPageDemo.Pages.Contacts.IndexModel
@{
}
<h1>Contacts/Index page</h1>

result: