A state may elect to have a variation from a standard in the Annex for a period of 6 months, thereafter it is required to notify all signatories of the variation from the standards. For day to day, the Jeppesen ATC pages for each country give a good breakdown of variations of rules. Each state is also supposed to establish a position on its Electronic filing of Differences matrix of compliance with ICAO standards (& recommended practices). Amazingly one of the best laid out versions of that is with the NZ CAA, almost a pleasure to work with their rules.
https://www.aviation.govt.nz/about-u...nization-icao/