For anyone not familiar, Yitang Zhang was something of a rags-to-riches story. He was working as a lecturer just ten or twenty years ago when he proved a major open conjecture in number theory. It's a result simple enough that you could explain the statement to a high school student if they've developed some intuition for integers, so he's gotten some attention from people who aren't full-on math nerds as well.
I've never been deep into number theory and can't really give details on it, but I can appreciate that solving an unproven conjecture that is possible to explain to a layman while still being something people care about is an absurd achievement, so doing that with a full-time job that does not involve research is even moreso.
Questions that are both interesting and easy to ask are questions that have endured many minds spending aeons looking at them from different angles without being cracked. The problems that require years of study to grasp are the easy ones. These "easy ones" go into an unfathomably large toolbox for approaching the simpler statements that have resisted time, there is almost zero chance of someone proving a layman conjecture without an extensive knowledge of those tools.
Obviously he has a phd, but keeping up with the background needed to approach something like that with a full-time job on the side is absurd. I passed a phd qualifying exam in algebraic topology about six years ago and I would fail exam 1 of an intro AT course if you had me take it today. This guy had managed to keep fresh with research-level knowledge well into his fifties when he proved that result.
I can't tell you anything about his discipline since I can barely tell much about my own (in my defense I am slightly less hopeless with my other qualifying discipline), but a few years ago Zhang was probably the most revered number theorist out there besides Terrence Tao and I'd assume that hasn't changed significantly since then.