If you have an existing web application, open it in Visual Studio Code (preferably copying it to a new development branch) and then use the PWA Studio tools to convert it into a PWA.

Not only can you integrate JavaScript with Blazor, that integration provides a strategy for moving your existing pages to Blazor without having to rewrite your existing JavaScript code.

An HTML Helper is a bit of Razor code that can be called from multiple places in a View. But, if you put your Helper in the right place, you can also use it from any View in your application.