Constant current in connectable LEDs

Something that's been intriguing me for a while, so it's time I found out.

LEDs should be run at a constant current to prevent runaway, yet you can get connectable strings of fairy lights which work fine whether there are 100 or 1000 leds. How does the driver do that? Does it detect how many LEDs are connected somehow?

I have a cheap flashlight with 24 LEDs all in parallel. Its battery voltage is higher than the LED voltage. Why don't the LEDs have current runaway and burn out? Probably for the same reason as the fairy lights:
The battery or power supply has an internal resistance that limits the current.

But the current for 100 parallel LEDs will be 1/10th the current for 1000 LEDs so there must be a constant current circuit to power them. Maybe the fairy lights are like strips of LEDs where there are 3 LEDs in series and in series with a current-limiting resistor and there are many groups like that.
"In the good old days" (20-30 years ago) constant current diodes were made. You could pick nearly any current you needed. Today you can make one from a voltage regulator IC and a resistor or with discrete parts.