Which UWB chip is recommended by Apple to be used, if one wants to build an interactive Airtags device? Is there a support for 3rd party tag vendors at all?
which chip is recommended by Apple for 3rd party "Airtags+"
On Apple Developer pages under "Nearby Interaction" section there is a list of recommended UWB Chipset Manufacturers: https://developer.apple.com/nearby-interaction/
You can also google "UWB Beacons" and get one of the available Developer Kits. They typically have the latest UWB chip that is compatible with FiRa and interoperable with U1.
After you test their Dev Kit you can contact the company and they might provide additional services if you need to design own tags, etc.