At the end of Terminator: Genisys we see Pops die after being flung out of the time displacement device:
How does he then show up again later in the movie?
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It's established early in the film that the poly-alloy can be used to repair the terminator. When the T1000 finds the "killed" young Arnie T800 you'll notice it drops some of itself into the head wound. This brings the T800 back to functioning again to chase Kyle Reese.
So the remains of Pops landing in a tank of polyalloy was more than fortuitous.