how did they get the prison realm back from kenjaku

They didn’t really get it back from Kenjaku by force — Gojo’s allies recovered the Prison Realm after the Shibuya Incident, and the key step was using the back of the Prison Realm to open or undo the seal. The Prison Realm’s back had to be located and handled as the way to free whoever was sealed inside, and Kenjaku had previously hidden it deep in the Japan Trench with seals and cursed spirits guarding it.

What happened

• Kenjaku used the Prison Realm to seal Gojo during Shibuya.

• Later, he stored the back of the Prison Realm in the Japan Trench as a fail-safe.

• Gojo’s side eventually obtained the back and used it in the unsealing process.

The short version

The “return” of the Prison Realm was less a robbery and more a retrieval of the object’s back gate , which is what made freeing Gojo possible.

If you want, I can also give you the exact manga arc-by-arc sequence of how they found it.