I believe a lot of times in the US, those companies become public utilities [0], which are publicly owned through stock (or privately owned)[1] but are highly regulated by the government and also I think exempt from laws prohibiting monopolies[2]. I'm not sure what's the process of a company going from a regular public company to a public utility though.

[0]: https://en.wikipedia.org/wiki/Public_utility [1]: https://marketrealist.com/p/are-utility-companies-public-or-... [2]: https://www.law.cornell.edu/wex/public_utility