How do people just go missing...?

How do people just go missing? There are so many missing people in the world…. every single day….. How does it happen? It’s a little scary people just vanish into thin air never to be heard from again… Are they mostly kidnapped or murdered? Maybe they become runaways? It’s a little bit scary…. I’m a female and go to places by myself a lot, or walk alone, go to the store by myself, etc. What are ways I can avoid going missing or being in danger? Sorry I know this is kind of a morbid question.