Site health alerts that I should remove inactive themes, but every time I update WordPress, the default themes are automatically installed again. How do I get rid of automatic twentywhatever themes installation?
It is wise to have at least one standard theme installed, because it will always work. If your current theme, for whatever reason, no longer works with the current WordPress install, you will have a backup theme to get your site running again.
That said, you are not the first to complain about this and you can stop it by adding this to your
define('CORE_UPGRADE_SKIP_NEW_BUNDLED' , true);