I bought one of the Xiaomi smartbands, which also have FreeRTOS and allow you to customize the watch faces. The format in which the spheres must be created in order to be published on the smartBand is documented.
So I have two options.
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Create my own app and publish it on the Galaxy Store, where I can create my watch face designs and upload them to the Fit 3.
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Request permission from Samsung to publish the watch faces you design within the Galaxy Wear app.