blazor_main.cs

public class MainLayout : 
    BlazorComponent, ILayoutComponent, IComponent
{
    protected override void BuildRenderTree(...)
    {
        base.BuildRenderTree(builder);
        builder.OpenElement(1, "div");
        builder.AddAttribute(2, "class", "container-...
        builder.AddContent(3, "\n    ");
        ...
        builder.OpenComponent<NavMenu>(10);
        builder.CloseComponent();
        ...
        builder.AddContent(0x10, this.Body);
        ...
    }
    
    public RenderFragment Body { get; set; }
}