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How to turn on uploading your photos to Google cloud:
at a random moment (maybe a fraction of a second before you clicked on your photo) a dialog box appears asking you whether you want to upload your photos to Google cloud: the options are "yes" and "keep nagging me in random intervals later", clicking outside of the dialog also counts as "yes" (so maybe you have 'consented' without even having an opportunity to read what the text says)
How to turn off uploading your photos to Google cloud:
open Google Photos app on your smartphone
click on your profile photo
click "manage Google backups"
change the switch to "do not backup"
open Google Photos web page in your browser
select all photos
click "delete"
go to the recycle bin
click "delete"
I am happy that I have an AI to figure this out. When you already know how to do it, it seems obvious in hindsight, but if you previously didn't even know that there was such a thing as a Google Photos web page (because you only knew the smartphone app), it was much less obvious.