Will there be an ending? Most likely, yes. If that note about "alternate ending(s)" is anything to go by. All things subject to change of course, but there might be plans for it.....
How will it end? Only the developers know. Your guess is as good as anyone else's.
Endings in these kind of crafting games have never really been that good since these kind of games never have that much story to connect you to the main villain (Minecraft and the Ender Dragon, Terraria and the Moon Lord even though that was pretty cool). Subnautica, however, through many PDAs, locations and story elements, I think this game will change that with a somewhat satisfying ending. The most satisfying in my eyes would be making peace with the Precursors, freeing the Emperor Leviathan, having the Precursors fix the Aurora ready for you to leave whenever you want (to end the game). But maybe that isn't where they're going with it. I'm certain it will have something to do with the Emperor, but I don't know exactly what. It will have an ending though, definitely
It's a sandbox, it doesn't need an ending. The ending is when you've beaten all the content and explored everything and decide hey this was fun let's do something else.
If you want to "end" and "beat" the game there will be a rocket that you have to build to leave. That is the plan and what the devs are working on right now. It will also be entirely optional of course, just as the story is.
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How will it end? Only the developers know. Your guess is as good as anyone else's.