I'm a dizzy daisy getting lazy.

Ash: My hands are Rated E for everyone!

Shadow the Hedgehog and Mewtwo trauma bonding not only over the dark entities that took over their body but also their dead friends

Where last we saw Mewtwo, on Cero Island in a sanctuary of pokémon he rescued, he probably had an off-screen temporary field-lab in imitation of the one on Mt Quena. "Temporary" and "field" still of course mean that he's got advanced medical tech, supercomputers (possibly with untraceable internet access*?), etc that he either assembled in a cave from whatever scraps and local supplies he has on hand (plus possibly some amount of reality warping to conjure things up?), and which he can easily disassemble or teleport with the sanctuary.