It’s determined by the governments. The current daylight saving time dates in the US were changed to what they are by George W Bush a number of years back to have more DST days.
Not every country changes their time (and the state of Arizona doesn’t either. Half the year they’re the same as Mountain time and the other half Pacific time).
Governments each set their own dates for switching between Daylight Savings Time and standard time. It could be determined by things like sunset/sunrise based on latitude/longitude of the country, crop seasons, holidays (US pushed back DST end to have it after Halloween, so more daylight to protect kids), etc.
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