Modify "head-placeholder" in html.html.twig
I suggest you look at https://www.drupal.org/docs/8/theming-drupal-8/adding-stylesheets-css-and-javascript-js-to-a-drupal-8-theme, which provides pretty good documentation on how to add CSS in a theme.

If you don't extend from a theme then you technically extend from a special theme called "stable". You should get almost no default CSS then, at least from core modules. Unless that really results in a problem, in which case there are some options to override it (documented above), I wouldn't worry about that.