And all of this starts with the data itself. It is the data you want to access which has the real value. Data you should own from the beginning.
If the data is in an open standard format, there is a possibility to break free.
If you cannot control the data, there are no baseline for digital sovereignty. If you cannot have access to software being able to make use of the data in a meaningful way for you, there are no baseline for digital sovereignty. If the software cannot be written, because the data format is unknown or too closely tied to the service provider generating the data, there are no baseline to achieve digital sovereignty.
With open standards, there can be built open source software using those open standards. Thus, you can decode and extract meaningful information from the data.
There are also no requirements anywhere that there must be more implementations for open source project from more countries. They key point is that source code must be open and available for all. That takes away the chances of someone talking full control of the software and restricting the freedom otherwise possible. Without a source code available, the path to extracting meaningful information ends up incredibly hard.
Open sourced software is one piece of the digital sovereignty puzzle, data in an open standard is another piece in the same puzzle.
Having access to the data files containing your information is yet another piece in the same puzzle. You cannot achieve digital sovereignty without all of these three pieces;then someone will still have control of your information.
Likewise, if you use a service with a proprietary API - you are bound to that service as long as that service uses the same API. If more service providers provide the same standardised API, you can more easily switch between services. Again, open standards is a key component for digital sovereignty - otherwise you will not be able to process your data as you want.