Why is _framework/blazor.webassembly.js not found in the deployed Blazor applications?

You have to update the virtual app path or app base path in index.html to resolve this issue.

That is, the hosted application expects the assets from root folder of the web server. So, you have to update the subfolder name in index.html to serve the files from the path.

For example, if I am hosting the application in http://example.com/blazor/, the index.html looks like the following.

<base href="/blazor/" />   

I would suggest you check the below link for better understanding:
https://docs.microsoft.com/en-in/aspnet/core/host-and-deploy/blazor/client-side?view=aspnetcore-3.0#app-base-path