How did they get that ring like shape? And will they vanish in many years?
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They are (mostly) atolls. What you see today is the rim of old volcanoes, generally stabilized with coral reefs. The volcano went dormant before they got too high or big, and the central crater floods.
Given that most of the islands are only a couple of meters above sea level, yes, it’s likely they’ll largely disappear in the not too distant future.
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A volcano builds up an island. The volcano becomes dormant and begins to erode. While it does, coral builds up around it as a fringing reef (like Moorea) The volcano continues to erode until it separates from the fringing reef, leaving a lagoon with a central island (like Bora Bora). At that point we call it an atoll. That central island continues to erode until the coral ring is all that remains, like the Marshall Islands or the Tuamotus.
That’s also why you only see them in tropical locations, because coral buildup is the necessary mechanism for their formation.
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