ICANN (Internet Corporation for Assigned Names and Numbers) is the organisation that oversees the internet. Together with stakeholders from all industries and all corners of the world, ICANN sets the rules.
One of those rules is when new domain extensions may be applied for; another one is who is allowed to operate a domain extension. All this is highly regulated.
The last time new domain extensions could be applied for was in 2012. Almost 2,000 parties did so for a total of over 1,200 different extensions. Since then, there has been talk of a new round of applications. It’s most likely that this new round will be in 2026, fourteen years after the previous round.