The left arm

We know that the Miyao we see in phase 1 is not the real Mitake Miyao but a program made to resemble him, the real Miyao is already dead at the start of the game. The Replica Miyao is a murder program made to bring war and destroy the world.

We see the vision of Miyao's corpse split in two, with only the <left> half of his body remaining.

A perfect corpse examination, confirming identity and making death certain, 'No human could survive with his head destroyed'.

We know Miyao is dead and we also have seen his death, he is the child who died during the filming of the propaganda video in the prologue.

Miyao's sleeve and gauntlet

What happened is most likely that Miyao opened his rejection shield only halfway through, protecting half of his body - the left half which is the half that has the gauntlet on.

And.. the half that has the selcom on -- the left upper arm to be exact

The Selcom can store everything as data, even memories and knowledge, this is the key to how Miyao  program was created. Talking with himself, Miyao argues that he is Miyao because he has the memories of Miyao, however as he says having memories doesn't mean that they are yours.

Miyao program was created from the memories stored on the real Miyao's selcom, the Data fragments are some of these memories. However the selcom must be put into someone else in order to work.

"We are one flesh", because Jayden has implanted himself with Miyao's <left> arm.