The final Cart of Chapter 5 is the Rusty-Gold Cart. It's found guarding the iced over archway near the "Cooling Slope" Shrine, which you can reach by following the path beyond the Mother Of Flamlings arena. However, the Rusty-Gold Cart will only spawn if you've followed certain specific steps in order. Here's what to do to spawn the Rusty-Gold Cart:

Defeat and speak to the Pale-Axe Stalwart.
Defeat the Brown-Iron Cart. https://youtu.be/ze8nW03mn40
Defeat the Gray-Bronze Cart.https://youtu.be/OZ3n8MF1GYQ
Speak to Ma Tianba inside the broken down Cart in the lava past the "Valley Entrance" Shrine. https://youtu.be/ZMvP7-Wd7FM
Defeat the Crimson-Silver Cart. https://youtu.be/9HXhKBk2ksQ
Speak to the Pale-Axe Stalwart again at "Ashen Pass I".
Speak to the Pale-Axe Stalwart again where you fought the Crimson-Silver Cart.
Approach the iced over archway near the "Cooling Slope" Shrine to spawn the Rusty-Gold Cart. https://youtu.be/xJhAxcDRgM4 (How to get here)

If you complete all of these steps, then as you approach, a cutscene will trigger, showing the Rusty-Gold Cart having just wiped the floor with the Pale-Axe Stalwart. This is the toughest Cart of all, with a lot of mobility and health to whittle away at over time. A Flock Of Many, Immobilize, and Charged Heavy Attacks are still your best bet here. Don't expect the Rusty-Gold Cart to stagger; just keep circling round to its back and preparing to dodge its flame gouts.

As a reward for defeating the Rusty-Gold Cart, you get the Flame Orb Curio, and you can also speak to the (now-dying) Pale-Axe Stalwart, after which you can finally open Chapter 5's secret area: Bishui Cave. Just go up to the iced over archway and interact with it to reveal the secret area, which contains three more bosses to face.

Feng-Tail General Boss [Chapter 6] - Black Myth Wukong

Son Of Stones Boss Fight [Chapter 6] - Black Myth Wukong

Gold Armored Rhino Boss Fight [Chapter 6] - Black Myth Wukong

Jiao-Loong Of Waves Boss Fight [Chapter 6] - Black Myth Wukong

Cloudtreading Deer Boss Fight [Chapter 6] - Black Myth Wukong

Chapter 6 "Unfinished" Verdant Path, "Supreme Inspector" Boss Fight - Black Myth Wukong

Chapter 5 Ending Cutscene - Black Myth Wukong

How to Beat the Yaksha King – Boss Fight Tips - Black Myth Wukong

Yaksha King Boss Fight - Black Myth Wukong

Yaksha King Boss Fight and Cutscene [Chapter 5] - Black Myth Wukong

Top Takes Bottom and Bottom Takes Top (secret boss) - Black Myth Wukong

How to unlock Rusty-Gold Cart (secret boss) and find "Bishui Cave" Area - Black Myth Wukong