Blazor application renders UI as a HTML content in client-side (browser). So, based on the width of the viewport in a device you can determine if it is a Desktop or a Mobile device and thus design responsive web pages accordingly.
The meta tag named “viewport” is used to design the webpage responsiveness and this is included in all Blazor applications, _Host.cshtml file in Blazor server-side application and index.html file in Blazor Web Assembly application. CSS styling makes designs responsive in Blazor apps.
For more details, refer to this link.