Which is the entry file used by vue-cli-service?

vue-cli-service uses Webpack with a default configuration of

entry: {
  app: [
    './src/main.js'
  ]
}

This can be altered in vue.config.js if you wish. See https://cli.vuejs.org/guide/webpack.html#simple-configuration

Webpack will build a JS bundle, starting at the entry then inject that into the index.html file and that's how your app starts.

You can see the entire configuration for your app using

vue inspect

See https://cli.vuejs.org/guide/webpack.html#inspecting-the-project-s-webpack-config

It is hardcoded in @vue.

Relative Path: node_modules/@vue/cli-service/lib/config/base.js

Line 28-37:

    webpackConfig
      .mode('development')
      .context(api.service.context)
      .entry('app')
        .add('./src/main.js')
        .end()
      .output
        .path(api.resolve(options.outputDir))
        .filename(isLegacyBundle ? '[name]-legacy.js' : '[name].js')
        .publicPath(options.publicPath)