Once they identify a familial hit they have it narrowed down to a relatively small list of possibles. Now do the detective work to exclude people - deceased, not living in the area, etc.
Once they identify a suspect they can obtain DNA either voluntarily, by court order, or surreptitiously (follow the suspect and collect soft drink cups etc.)
The way authorities caught The Golden State Killer was nothing short of amazing
https://en.wikipedia.org/wiki/Joseph_James_DeAngelo#Investigation
On June 15, 2016, the FBI released further information related to the crimes, including new composite sketches and crime details;[130] a $50,000 reward was also announced.[165] The initiative included a national database to support law enforcement's investigating of the crimes and to handle tips and information.[166] Eventually, "through the use of genetic genealogy searching on GEDmatch, investigators identified distant relatives of DeAngelo — including family members directly related to his great-great-great-great grandfather dating back to the 1800s.
Based on this information, investigators built about 25 different family trees. The tree that eventually linked to [DeAngelo] alone contained approximately 1,000 people. Over the course of a few months, investigators used other clues like age, sex, and place of residence to rule out suspects populating these trees, eliminating suspects one by one until only DeAngelo remained."[16]
During the investigation, several people were considered and later eliminated as suspects:
Brett Glasby, from Goleta, was considered a suspect by Santa Barbara County investigators. He was murdered in Mexico in 1982, before the murder of Janelle Cruz; this eliminated him as a suspect.[167]
Paul "Cornfed" Schneider, a high-ranking member of the Aryan Brotherhood, was living in Orange County when the Harringtons, Manuela Witthuhn, and Janelle Cruz were killed. A DNA test cleared him in the 1990s.[168][169]
Joe Alsip, a friend and business partner of the victim Lyman Smith. Alsip's pastor said that Alsip had confessed to him during a family-counseling session. Alsip was arraigned for the Smith murders in 1982, but the charges were later dropped,[170][171] and his innocence was confirmed by DNA testing in 1997.[172]
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