Splendidus is a small set of private, self-hosted tools run by one person for their own household: a mail and calendar hub, a single sign-on page for those tools, and a personal health dashboard.
It is not a public service. There are no sign-ups and no accounts for the public. Some of these tools use “Sign in with Google” so the owner can connect their own Google account (for example Gmail, Google Calendar or Google Health data) to tools running on their own equipment.
How Google data is handled is described in the privacy policy; use is governed by the terms.