The theme pulls in github.com/FortAwesome/Font-Awesome as a module. You can see this in the go.mod file in the project root. When you build a Hugo site, the module dependencies are placed in the module cache. You can either dig deep into the cache’s directory structure to see the files, or run hugo mod vendor to vendor dependencies into the _vendor directory in the root of your project. You could leave the _vendor directory as is, but usually, it’s better to delete the directory so that you can update it via hugo mod update.
No. Your project configuration is the config directory in the root of you project—you can do it either way.
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