How to run an external program on a database used by a website? [See thread for more]

What I mean is, giving an example:

Facebook uses AI software to "take data" from user contents, friendships and so on. This is a huge time-and-resources consuming task and it's not possible to be run on user clicks, neither make users wait for a possible response.

So... How they do it? Do they have a simple background process that runs pretty much forever? How it works?

PS: I know it is probanly a very technical answer, but try to explain :D