It's done by software using an LLM, not just a raw LLM. They do only work with text, but you can get it to output the text "get_weather(mylocation)", and instead of just outputting that directly to the user, the software running on top of the LLM runs a " get_weather" function that calls some weather API. The result of that function is then output to the user.

Any time you see an "AI" taking "actions", this is what happens in the background for every action.