Responsive Search Ads (RSAs) replaced Expanded Text Ads. You provide up to 15 headlines and 4 descriptions; Google combines them dynamically. Writing for RSAs requires a different approach than old-style ads.
Don't write 15 variations of the same thing. Cover different angles:
Google may show your headline without the others. Each must make sense on its own, not depend on being paired.
30 characters is tight. Cut every unnecessary word. "Get the Best Deals on Running Shoes" (34) fails; "Running Shoe Deals - 40% Off" (28) works.
90 characters each. Use them to:
Pin a headline to position 1 if it's critical (brand name, required disclaimer). Otherwise, let Google optimize. Heavy pinning removes the algorithm's ability to optimize.
Google rates your ad "Poor", "Average", "Good", or "Excellent". Target Good or Excellent. Higher Ad Strength correlates with better performance.