CityStrides gets its country/region/city/street data from OpenStreetMap.
Cities are on an automatic syncing schedule that attempts to sync batches every minute, assuming the current system load is not too high. Each city displays its last updated date on its page, just under its name. The icon next to the date spins while the city is syncing.
There is no set/known frequency of city updates, since the system is constantly operating and being affected by many things (size of updates, new cities being added, people running in new cities, people deleting activities, unreliability of Overpass servers, etc).