When it's more than one company working together in a monopoly-like fashion, the term is "oligopoly".
https://www.merriam-webster.com/dictionary/oligopoly
There is another word for it: cartel.
e.g., the Phoebus cartel https://en.wikipedia.org/wiki/Phoebus_cartel
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There is another word for it: cartel.
e.g., the Phoebus cartel https://en.wikipedia.org/wiki/Phoebus_cartel