Kenpachi gets his arm back through Mayuri Kurotsuchi’s medical/experimental restoration , not by simple instant healing. In Bleach: Thousand-Year Blood War , his arm is damaged badly in battle, and later it’s restored after Mayuri intervenes.

What happened

  • Kenpachi loses the arm during the fight with Pernida.
  • Mayuri recovers and restores him using a special capsule/pod-like medical process.
  • After that, Kenpachi is shown with the arm back, though he later suffers more damage again in the story.

Why people get confused

There are actually more than one arm injury in Kenpachi’s fights, so fans often mix them up. One injury is from Pernida, and another happens later when his Bankai pushes his body too far.

Simple version

He didn’t “regrow” it naturally in the usual shonen sense — Mayuri restored it after the injury.

TL;DR: Kenpachi’s arm comes back because Mayuri repairs/restores it after it’s destroyed in battle.