Based on phone Breakdowns. Iqoo seems to be using a single layer Vapour chamber with a surface area of 6000 mm squared.

While OnePlus 12 uses a double layer vapour chamber with a surface area double that of Iqoo dual to 2 layers. 

Although on paper it may appear that More surface area means better cooling but we all tested that iqoo 12 runs much cooler than OnePlus 12 because using double layer in OnePlus 12 vapour chamber traps heat and dissipation is slowed down. Whereas Iqoo single VC doesn't trap heat and can easily dissipate.