Big platforms’ ad libraries or how to check which ads your competitors or any other page in the world are running:
Google: https://adstransparency.google.com/
Facebook/Instagram: https://www.facebook.com/ads/library/
LinkedIn: https://www.linkedin.com/ad-library/
TikTok (for top ads): https://ads.tiktok.com/business/creativecenter/inspiration/topads/
X (fka Twitter): https://ads.twitter.com/ads-repository
Pinterest: https://ads.pinterest.com/ads-repository/