Gadgetbridge is an Android free and open source application to manage data from wearable devices in a privacy-friendly way. It supports a lot of wearable devices like amazfit, mi bands, fitbit, etc.

Its strength is that the data registered by the device remain yours and under your control, without beeing used by large companies to make money. It resembles Exist’s philosophy. Data can be exported periodically on the phone in an automatic way.

No API, not possible.