In order to comply with GDPR, Garmin allows users to download all their personal data, recorded activities, and a summary of all Garmin connect data as a large zip-file.

The zip-file can be requested at: https://www.garmin.com/en-US/account/datamanagement/

Most data inside is stored in a json-file.  Activity and HR-data is stored in binary fit-files (harder to access).