What is the best Amumu build in Wild Rift?

Amumu's most reliable build remains a coherent tank base around Sunfire, then an adjustment between sustained damage and targeted resistance depending on the enemy draft. The right build is the one that still lets him hold after the engage.

Is Liandry’s Torment mandatory on Amumu?

You mainly adapt when the problem is no longer hitting multiple targets, but staying alive long enough for them to be punished. If poke chips you before the objective, if burst breaks the engage, or if enemy AP melts you, you need to move up the useful defensive answer.

Which enchant truly helps Amumu when he mainly has to secure access or last after ultimate?

Protobelt is often the most versatile for reliable access, while Gargoyle becomes better when you are already landing good engages but need to last longer after impact.

Amumu Wild Rift Best Build Guide (Patch 7.2a)

Amumu’s default build works because he rarely wins fights through a single burst window; he wins them by locking down space, maintaining contact, and punishing enemies who remain grouped around him.