There are many triggers which provide ideas. Snatches of overheard conversation, snippets in newspapers, articles in magazines, non-fiction books, various situations in our daily lives and those of friends and acquaintances.
All of us have access to these triggers, but some people merely find them interesting other people use them to write fiction. Sometimes ideas stay with us for a long time. Again and again we return to them and eventually write the first line of a short story or novel. We have grasped the trigger and developed it by asking ourselves: What happened? Where did it happen? When did it happen. Why did it Happen? How did it Happen? Who did it happen to?
At a party hosted by another fiction writer, a gentleman followed me around telling me at length that he had many ideas and wanted to write a novel. Eventually, desperate to be rid of him, I said. "I'll tell you how to write one." He thought I had a magic formula but I told him. "Write the first word and continue until you reach the end." Good advice, although I say so myself, we never know where our ideas will take us.