Clearing the cache of third-party apps varies from developer to developer. As seen below, some like Slack offer an option in the app preferences in iOS Settings, while others like Facebook don’t. Check out the settings of your apps to see your options.
For apps that don’t provide a clear cache button, deleting the app and reinstalling it is the next option.
You can always take a look at Settings → General → iPhone Storage to prioritize which third-party apps you may want to tackle first.
