[Faaak]

These are called ancillary services. There are 3 levels: frequency control (for example dams), primary reverse (fast acting power, in less than 10s), and secondary reserve (slow acting: < 15min).

Participants can either be positive (they consume more energy, for example PHES systems), or negative (they inject or consume less energy).

All these ancillary services are paid (annual auction + per case). Nowadays, it's getting bigger with VPP: virtual power plants, which aggregate small loads (i.e. small ~1MW generators) in order to propose a bigger load to TSOs.

It's all automatic

If you're interested: https://www.swissgrid.ch/en/home/customers/ancillary-service...

[Faaak]

On top of that, newer solar inverters MUST (at least in the EU) reduce power if frequency rises in order to automatically shed power in cases of extreme supply