Investigators predominantly search what are termed public record databases. These are databases that store personal details for persons resident in a particular jurisdiction and are accessible to anyone. An example of a public record database is the ABN Lookup,
"the public view of the Australian Business Register". According to the site, "It provides access to publicly available information supplied by businesses when they register for an Australian Business Number (ABN)." It is possible to search this register under the name of a person or a business for free.
If there is a listing on the ABN registry, you might get bits and pieces of information about the registrant that can include name, postcode, trading name, business name and an indication as to when such details were current. The registry occasionally reveals the actual, current place of residence of a person but, if there is a listing, usually just gives an investigator a piece of the puzzle. A thorough investigator will typically search through roughly 10-15 or so databases, some of which are free and some of which require subscriptions and payments. It is usually necessary to cross reference the information obtained from each database to intuitively build a profile of the person sought as no one source can be considered definitive or comprehensive.