isn't the software literally a joint effort between Samsung and google?
Yeah, but I am sure there are limitations imposed on both sides on how much access they give each other to Closed Source Code. The more likely scenario (which is what we do with outside Dev's) is to create/grant access to API's that allows them to do thing that they need to do. However, if those API's are poorly documented or poorly implemented, there is only so much Samsung can do.
I am sure Google gave Samsung pretty deep level access in order to get their flagship watches ported over to WearOS over Tizen, but access only goes so far. From what I understand, Samsung's primary implementation is more of a "Skin" for WearOS to make it look and feel like their past devices as much as possible, but not given a deep level of access to the "internals".