Welcome to the community, @sophey! Please be informed that the https://kf.kobotoolbox.org/ is the KoboToolbox Formbuilder/frontend interface (KF). This is the part where you design forms, manage your projects, monitor data collection, and download data. It’s what you interact with as a user (dashboard, project settings, submissions, etc.).
https://kc.kobotoolbox.org/, on the other hand, is KoboToolbox KoBoCAT (KC) or data server endpoint. It mainly handles the storage and management of your data. This is where the API for submissions and data retrieval operates. It’s also what your form collectors (e.g., KoboCollect app, Enketo webforms) communicate with when submitting data.