system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a digital camera "know" what to focus on?

I tried to find similar answers but none that hit the nail on the head as to how the camera "knows" what to focus on. What I mean by this is how does my camera know to focus on a specific person or thing, when there are countless other things it could focus on.

Say, a picture of a person with a stop sign behind and some trees also, how does it recognise the person and ignore the sign and the trees?
gpt
There are two methods for cameras with autofocus...

Focusing on the object in the  center or at a preset focus area. This is determined by the photographer and/or camera. 

The newer method uses machine learning for subject detection and selection. What is the photographer trying to take a picture of? Is it human? Does it stand out more than other objects? face detection, etc. Sports photography is slightly different, it’s not machine learning, but uses the chip to track objects once selected so you can burst while the subject is moving and maintain focus.

As far as how it actually focuses, there are a couple methods like phase detection or contrast detection of the subject once the camera or photographer has identified the subject.