I have to ask, did you search the forum? Because the answer is there and it isn't that long since it was discussed.
There are multiple ways to get the annotations from the device into other storages. Including a built-in but not officially supported way to export them to a file on the device.
There are also ways to copy the annotations to other Kobo devices. But, they are manual and have problems. Most of the discussions have been about how to move everything when upgrading the device. One of the techniques mentioned could be used to copy for just one book, but, it would be a pain unless someone automated it.
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