What is the difference between Ad_group_ad and Ad_group tables?

I was asked to call Google-Ads API in order to create a report with these fields:

I went to ad_group_ad table.

I wanted to make sure I understand this table's meaning: Why isn't this just called Ad?

What is the difference between Ad_group_ad and Ad_group tables?

There were a few metrics which I couldn't find in ad_group_ad I couldn't filter the views which contain them (for example impression_share)

I know impression_share exists for keywords and campaign, so I found them in the relevant tables. So I understand they cannot be broken down at an ad level.

But I still couldn't find the other highlighted metrics even at a campaign level.

Here is my campaign query try:

SELECT    metrics.clicks,    metrics.impressions,    metrics.ctr,    metrics.cost_micros,    metrics.cost_per_conversion,    metrics.cost_per_all_conversions,    metrics.all_conversions_from_interactions_rate,    metrics.all_conversions_value,    campaign.name,    campaign.id,    segments.device,    segments.date,    campaign.advertising_channel_type,    segments.click_type,    campaign.geo_target_type_setting.positive_geo_target_type,    campaign.geo_target_type_setting.negative_geo_target_type  FROM campaign  WHERE    segments.date BETWEEN '2020-01-01' AND '2022-01-01'  ORDER BY    campaign.start_date DESC  LIMIT 100