A Blazor layout is similar to the ASP Webforms concept of a Master Page, and the same as a Razor layout in ASP MVC.
Almost every website on the Web has a template that is used either throughout the website (branding at the top of the page, copyright at the bottom), or throughout specific sub-sections of a website (such as a specific menu structure on the Admin pages of the site).
This is achieved by creating a view that acts as an HTML wrapper around the current page's content, the template contains a place-holder indicating where the wrapped page's content should appear.
This is the start of my reusable layout
Individual pages can then optionally specify a single layout it would like its content to be wrapped in.
-- Some way of indicating which template to wrap this page's content in --
This is the content of your embedded page
The resulting HTML would look like this