The only recommended integration with smartthings is by using a Hue bridge. You can mess up your zigbee network otherwise. And the hue bulb can only belong to one zigbee coordinator at a time.
See the community FAQ (the topic title is a clickable link)
If you buy a hue bridge you would be able to use the bulb with both smartthings and Alexa at the same time. But without one, you pretty much have to choose between one or the other, and as already mentioned, it’s just not recommended to use the bulb with smartthings without the bridge even though it’s technically possible.