No, they don’t share or play nice in 99% of scenarios.
Most of them do want you to remain inside their ecosystem. OuraRing doesn’t sync with Fitbit or Google Fit, and has limited to no syncing with Apple Health Kit now.
If you don’t want to build the backend of that type of integration, I built the authorization and API to pull the data into open source software. Our team built APIs for Fitbit, Google Fit, AHK, plus we’re working on Oura and Withings.
I have a team trying to publish it to Fast API for others to use but we have limited bandwidth.
Any interest or assistance is welcome.