On top of redesigned notifications that make them easier to identify, you can catch up with a helpful collection of your daily notifications based on a schedule you set on iOS15. The summary is intelligently ordered by priority, with the most relevant notifications at the top. However, by default, iOS15 will give you notification summaries twice a day; 8AM and 6PM local time. These default summaries might contain irrelevant notifications that you may find irritating. Setting up your own summary will flush out these unwanted notifications as well as times you'd like summaries sent to you. You can also completely turn off summaries.
How to set up Notification Summary
Open Settings > Notifications
Tap Scheduled Summary
Toggle the switch for Scheduled Summary if you haven't already
Select the apps you want notification summaries on
Tap "Add X apps" when you're done
Set a schedule and the number of summaries
Tap Turn on Notification Summary when you're finished
To edit which apps are shown in summaries, toggle their switches under "Apps in Summary"
You can turn off Notification Summary by toggling off Scheduled Summary in Notification Settings