Before explaining to techniques or tools. I will reiterate. Deep web and dark web are not the same. Ok, let me explain the deep web first, the deep web can be accessed by certain browsers to be accessed. However, its content cannot be identified, tracked or crawled by standard search engines like Bing, Google, Baidu because it is password protected or stored behind certain internet services. example. Chat, email messages and other. Dark web websites that can only be accessed via internet services such as The Onion Router (TOR) Content on the dark web is primarily hosted anonymously and is highly encrypted, providing an extra layer of protection against tracking and identification.

Then for some tools, you can use the OSINT technique to take advantage of the resources or datasets that you find. Investigation is not an easy thing. I will describe a few tools that you can use

1. Cybersixgill https://www.maltego.com/transform-hub/cybersixgill/
2. Pipl https://pipl.com/
3. Flashpoint https://go.flashpoint-intel.com/

You can take advantage of the Maltego transform, for actual techniques there are many but I can't name all of them

You can read article on internet or you can visit Maltego transform you can read it

Damn, i was noticed on my social network you was OSINT me haha nice try