Those purport to operate on the ions themselves, supposedly to favor one kind of crystallization pattern over others. In lab conditions, a permanent magnet is said to work equally well, presumably interacting with the ions as they flow past.
From what I have been able to determine, nobody has shown that such a gadget works with any reliability. I.e., it might work under certain circumstances, but there is no way to know if your water and pipes match such a circumstance without buying. And, the prices quoted seem badly excessive. There is probably not more than $5 worth of parts in there, if in fact there are any.
Personally, I would not buy one. You could experiment with a permanent magnet, but it would be hard to know if it was helping or making it worse.
A somewhat similar sort of gadget is supposed to actually work, on diesel immediately before injection into truck engine combustion chambers, to produce more complete combustion.
[0] https://www.homedepot.com/p/Scalewatcher-Nano-Electronic-Des...