How to override styles from child theme if all the styles are in a folder?

As long as your theme has been set up properly for child themes, it doesn’t matter how many style sheets there are or where they are located in the actual theme. The style.css in the child theme directory will prevail over them.

If it isn’t set up properly you can re-enqueue with a different priority.

If you can’t do that, then you can make your css selector more specific (ie, .header h1 instead of just h1) and/or add !important to the value.