How social media bots are taught to recognize faces?

Question

Social media

Recently, a lot of all sorts of bots for VK have appeared that can search for a face in a photo, apply effects, edit photos, and also analyze user behavior. How is this done in general?

Answer 1

look for a face in a photo

machine learning.
Photoshop has gone online.

Answer 2

OpenCV + simple (and not so) neural networks (Machine Learning)