If you're searching for people online, here are some go-to websites for a deep search:
Infortracer.com extracts data from numerous sites, databases and even social networks like LinkedIn or Facebook to uncover personal information like phone number, full name, current and past street addresses, email addresses, family members, social links, work history, photos, alumni info, relatives, and more. To narrow down a name search, input the city of residence.
Classmates is another US-only website for people who have an idea of the high school(s) the person they're looking for attended. You'd need their student name, state, city, and school to begin with.
LinkedIn is an alternative way of finding someone's email address, current and past jobs, education, qualifications, and more.
Usersearch.org is a reverse search tool that picks up individuals with just their username, email address, or phone number. It even focuses on covering cryptocurrency users with an active presence on forums.
TinEye is a very useful alternative to Google Image Search and if you're trying to find someone by only usingtheir photo. TinEye's reverse image search tool uses advanced image recognition technology that identifies people based on the images you upload.