They're probably just using an already trained model. They're almost certainly not training a model on-device.
AFAIK both the iphone and the pixel have a chip that are specifically designed to do forward propagation on a deep neural network. I don't know much about them though.
True - but what model would they want to use on-device?
All the feed ranking stuff is done serverside I assume... And apart from that, the app is just for posting and reading text and pictures. I don't really see any use for any ML there.
If one could profitably offload serverside CPU to the client then every app would be mining bitcoins on-device.
But it turns out that user complaints of a laggy app, hot phone and bad battery life (and therefore lost users and lost ad revenue) far outweigh any CPU time savings on the server.
AFAIK both the iphone and the pixel have a chip that are specifically designed to do forward propagation on a deep neural network. I don't know much about them though.