From what I understand the fact that the test had no name and contact details attached, they made the assumption that it could not have come through any official testing channels because the officials would have spotted it. So that left the home testing arena which is what i think they focused on initially. I haven't followed the search so not sure how they located him in the end.
It was explained in the last briefing. Like you say, they focused on the home testing, which meant using data from DPS, (delivery company for the tests), then matching that to some virus sequencing, (no, me neither), so were able to narrow it down from 10000, to 379, then one street. In a week.
Like you know when you sign for a parcel on those scanners? Well, the second the delivery lad pushes a button saying "delivered" on his scanner, the exact position he was standing at is recorded. Its called the Geo Code or sommet.