return DB::table('topics')->where('tags->tags', ucfirst( $tag ))->get();

tag_name     |     tag_seo_name
--------------------------------
Tutorials    |     tutorials
About us     |     about-us

return DB::table('topics')
        ->where('tags->tags', ucfirst( $tag ))
        ->join('tags', what to write here? tags->tags, ucfirst( $tag ), '=', 'tags.tag_seo_name')
        ->select('topics.*', 'tags.*')
        ->get();