Suppose someone on the 90 of 180 rule leaves Schengen on the 90th day.
Then less than 90 days later, they have a flight with one or more layovers in a Schengen country.
Does it get counted if it's the same plane? Change of planes?
What if it is overnight but you don't leave the airport?
How to calculate stays against 90/180 visa rules? relates but doesn't specifically mention layovers. If you don't get off the plane, I presume nobody even looks at your passport. But in one non-Schengen airport, I had to change planes, but first was required to go through passport control, claim luggage, and re-check the luggage.