Hello Poul Erik Mark Larsen
Please follow this article to install the quickstart. Install the Joomla 4 quickstart, then update your Joomla version to 5. It will work fine. Use PHP 8.1 or 8.0 while installation.
You do not need the JSON file if you install the quickstart. In the JSON file, template default settings is stored. If you customize template settings and wants the default settings back, only then use the JSON file contents in Template Options -> Advance settings. You have import the settings there. In the same way, you can keep a copy of your template settings (your style, text and other changes). Then you have to import it on your machine so that if needed, then you can retrieve it. More details are available in the documentation
Best regards