My SWAG is they are just switcher supplies, such as power your laptop.
A typical switcher has a line reactor to limit input current, then diodes & a cap.
That DC is then run to an inverter at 50-500 Khz {above human hearing..) and to a transformer. Since the transformer is at higher freq, it needs little iron & is thus cheap, lightweight and small vice a 60 Hz one. Say its output is ~15VDC. That different voltage DC is then filtered.
There is feedback; the output voltage is sensed, and the inverter duty cycle {fudging terms here} is varied to keep a consistent output voltage. Given enough range; it does not care about input voltage {or obviously frequency...}. You can have one unit that runs from >100VAC-250VAC in; 50/60 whatever Hz.
Switchers are tricky to design, picky [
VERY picky] about parts quality, [esp. the caps] and almost impossible to fix. So they were exotic & rare. BUT, then a few semiconductor houses started making IC's that did the hard work, and also provided design tools.
The result is products quite inexpensive to build in
large quantity, and usable around the world.
I expect a ballast is just a switcher designed for constant-current output, with whatever else is needed to start the lamps. [There are multiple ways to
start them. They'd be AC not DC output.
