A default layout can be specified in the /Pages/_Imports.razor file. This setting can be overridden by Blazor finding a more specific _Imports.razor file closer to the page it is rendering. The final (and most explicit) level of specifying a template to use is to literally specify it in the page itself using the
@page "/admin/users" @layout MainLayout <h2>Users</h2>
Note: You can have more than one _Imports.razor file.