How "buy now" act as trigger and drive into loop like "Hooks model"

Recently, I was trying to understand the 4 phases of the "Hook Model". Trigger, Action, Reward & Investment.

From the study, I found there is 2 main category of the trigger. Internal & External trigger. In most of the articles, I got some common examples that are "check" engine light, "play" button, "buy now" button act as a trigger. In other words, they are called CTA's(Call to Action). CTA's like "buy now" creates sudden impulse or urge to buy something. These types of purchases are known as impulse purchases.

Now, My question is How does a button act as a Trigger? I don't understand or feel any trigger while I stare at a button like "buy now". Or am I missing something here?