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.
As T-800 himself explained:
Polyalloy requires programming to take permanent form.
Without the CPU, it is harmless.
And T-800 does have a CPU, so this mishap acquired him a new body, similar (identical?) to the one that T-1000's have.
You may recall that he explains this; he didn't die, he was 'upgraded'. They never really specify what that means, but I believe it has to do with him being coated with the liquid metal.