Open source intelligence.

Gathering all information about a person or entity that THEY'VE made public online.

Think of reverse email and phone number tools. Those are two simple tools you can start with. Maybe even a free digital footprint tool online.

Get mellissa lookups on your phone. It's a reverse tool, it's on the playstore.

Also look up the OSINT community on reddit. They have a lot of tools and questions like yours that I think would be helpful for you to look at.