and in public_html/public/.htaccess that other, longer htaccess should be (edited)

this is where index.php and resources should be

and public_html might contain another dir called "public", so that's public_html/public

public_html is your silverstripe root, where all modules, themes and vendor (composermodules) are in

No its added to public_html/.htaccess (edited)

don't you add it to the wrong .htaccess...?