I need to launch writer, lookup a txt file and insert txt into writer template.
And where does the browser enter the scene?
Any file system browser should enable you to locate the Writer executable or the links to it, and launch it if you have “run rights”.
If you meant a web browser, most of them cannot launch other applications easily. The usual operation which looks like “launching an app” is when you click a link to open a non-html resource. Technically this makes your operating system (through its default file browser) launch the application which has been defined as the default handler for that resource. It looks like the web browser starts the external application.
Running a local application directly from a web page raises security and privacy issues, so in general, web browser developers are quite conscious about not implementing support for this. There used to be a solution using Java, with Java WebStart (JWS) enabled for the web page and a Java “hook” app installed locally to launch your application. I don’t know whether that will still work.