What is the difference between ad_group_ad and ad?

Question

Google Ads

Help me understand the difference. As I understand the structure:
There is an advertising company, it has groups ( AdGroup ( https://developers.google.com/google-ads/api/field... )) and ads ( Ad ) must be added directly to these groups .
If so, what is the role of ad_group_ad ( https://developers.google.com/google-ads/api/field... )?

And I still don't understand the difference between Ad and Asset . How are they related to each other if they are not the same thing?