Homelab Health is a private, personal app. It is not a product and is not offered to the public.
It connects one person's Fitbit data, through the Google Health API, to a training programme run by a family member. With her permission it reads her activity, sleep, health measurements and food log so her training can be adjusted, and it adds the gym workouts and body-composition readings she records elsewhere to her Fitbit so everything is in one place.
No one else uses it, and there is nothing to sign up for.