How do you configure and set the layout page in Blazor?

Technically, a layout is just another component.

To convert a component into a layout:

• Inherits from LayoutComponentBase, which adds the Body property to the layout.
• Uses the Razor syntax @Body in the markup where the content should need to be rendered.

The default template of Blazor contains MainLayout.cshtml under the shared folder. It works as a layout page.ecuted when the rendering of all the references to the component is populated.

@inherits LayoutComponentBase

<div class="sidebar">
    <NavMenu />
</div>

<div class="main">
    <div class="content px-4">
        @Body
    </div>
</div>

Use layout in component

The layout can be defined by using both @layout directive and Layout Attribute in a component.

Using @layout directive

[Layout(typeof(MainLayout))]
public class BaseComponent : ComponentBase
{
    public string text { get; set; } =
       "Hello!";
}

Define Layout globally

The layout can be defined globally for all components. So that there is no need to add them to each page.

In _import.cshtml file, import the MainLayout.

@layout MainLayout
@using Microsoft.AspNetCore.Components

I would suggest you check the below link for better understanding:
https://www.c-sharpcorner.com/article/working-with-layout-page-in-blazor/