OSINT or Open-source intelligence is the collection and analysis of data gathered from open sources to produce actionable intelligence. Wikipedia

It also involves methods, tools, and practices for gathering open-source information that can lead the researcher to the intended target via associated persons, entities etc.

Two books on the subject that I have in my collection are:

How to Find Out Anything

And Sources and Methods for Investigative Internet Research

https://read.amazon.ca/kp/embed?asin=B00HDO33SW&preview=newtab&linkCode=kpe&ref_=cm_sw_r_kb_dp_Y0E88HX1W3QKG0N8YBGN