Feature Flags is a concept that helps to enable or disable a feature on an app. Example you would want to roll out a new button on app for a certain set of users, A naive and simplified implementation would look like something below

flags = server.getFlags() // Fetch/Compute feature flags

if (flags.FeatureA == true) {
// show button
}

Insomnia user here too, I've found it to be simple,clean and to my taste.