The iPhone Photos Memories section is curated by a "machine-learning" algorithm that selects photos from your photos library. It does so when your iPhone is connected to a power source and it is not being used (Sleep mode).
Currently, there is no way to edit or override this algorithm and to select which photos are selected in the Memories and Featured Photos section of the For You tab of Photos. But you can certainly delete Memories and Featured Photos. (see link below)
The Photos Widget app in iOS 14 selects photos from these two categories - Memories and Featured Photos.
ProtectiveFather wrote:
How to I immediately, and permanently (a) turn it off, and (b) ensure that new products from Apple never ever again access my photos?
(a) You can delete Memories and Featured Photos to prevent these photos from displaying in your Photos Widget. But this means you will have to keep monitoring/deleting these Memories/Featured Photos OR you can remove the Photos Widget altogether.
Use widgets on your iPhone and iPod touch - Apple Support
https://www.iphonelife.com/content/how-to-remove-featured-photos-iphone
(b) New products like apps will have to be granted permission by you, so that they can access all or some of the Photos in your iPhone. The Photos widget feature is kind of subset of the Photos app itself, so there is no explicit permission required in this case.
Control access to information in apps on iPhone - Apple Support