1. Have you ever spoken to the people? They live next door.
=> Have you ever spoken to the people who live next door ?
2. It is a book. It will interest children of all ages.
=>  It is a book which will interest children of all ages
3. The people  keep having all night parties. They live next door.
=> The people who live next door keep having all night parties
4. These are the keys. They open the front door and back door.
=> These are the keys which open the front door and back door.
5. Here's an article. It might interest you.
=>  Here's an article which  might interest you