See https://vivokey.com/apex/ for general info and flexsecure-applets/docs at master · DangerousThings/flexsecure-applets · GitHub (also check the /applets directory) for developer information.
Other than the Vivokey apps, there are no applets yet certified for the Apex. There might be some in the future, and there will be more to come from Vivokey as well.