How do conspiracy theories get started and why do so many people believe them?

I have had long talks with my apartment manager who believes several conspiracy theories as fact. While I respect her opinions, some of her ideas border on lunacy. Is it possible that President Biden does not live in the White House? She thinks he travels from another residence for all his appearances. She showed me a picture taken in the White House of something showing 2017- for President Trump's term. Her conclusion is that Donald Trump is still President of the United States.