Apple - Where did this iCloud profile avatar come from?

I asked her (without giving away why) if she’s used this as a profile picture elsewhere and she told me it’s the pic on her YouTube profile. I added her Google email address as a secondary email (for account recovery) when I created her iCloud account, so Apple must have scraped her Google account for a profile picture.

The person signed in to iCloud can upload any image they choose and no avatar is chosen by Apple. On macOS there are some generic account photos so your question makes great sense, but iCloud has no such stock content supplied.

To date, everything is a digital file that someone uploads from the photos app or loaded with intention in to the iCloud control apps (settings / system preferences) or web site.

Tell your daughter she has good taste and I’ll encourage you and your son to start having fun with art or photos that reduce to a set of pixels.