Where a cold joint actually shows up on a job
A cold joint is not something anybody plans into a pour. It shows up when concrete already in the forms starts to set before the next batch reaches it and gets worked in against it, so the two never really become one piece. On a big pour, that usually traces back to timing between truck loads, a delivery running late, a mixer stuck behind the same traffic everybody else is stuck behind, or a pour that gets interrupted partway through by weather or equipment trouble. A driveway poured start to finish in one session with steady deliveries does not have this problem. A pour that stalls partway through does.
How a crew keeps the seam from forming
The fix is mostly about keeping concrete moving. Loads need to arrive close enough together that each new batch goes in against material that is still workable, not material that has already started to stiffen up, and the edge of what is already down gets worked back into the new concrete with a vibrator or by hand before that edge sets. Hot weather shortens the window, because concrete sets faster in the heat, so the margin for a late truck gets thinner on a summer afternoon than it is in cooler weather. On a pump job, the same logic applies to the pump itself. A stall at the hose, a section of line that plugs and has to be cleared, costs the same kind of time a late truck does, so keeping the line moving matters just as much as keeping the trucks on schedule.
What it looks like once the slab has cured
A cold joint leaves a visible line where the two batches meet, and because the bond across it is weaker than concrete poured all in one go, that line is where water tends to find its way through and where a crack is more likely to open eventually. On a driveway or patio, that is mostly a cosmetic and long term durability concern. On anything holding back water or carrying real structural load, a foundation wall, a retaining wall, it matters more. Not every visible line in a slab is a cold joint, though. Control joints and construction joints are both cut or formed on purpose, and they can look similar to an untrained eye. If a line in your concrete does not match up with where you know a joint was planned, that is worth a call rather than a guess.