My iPhone 11 Pro max has 2 IEMIs, however, I can't find 2 SIM slots, Can anyone pls help how to use 2 SIM cards ?

If it is an iPhone 11 Pro Mac that was originally sold in China Mainland, Hong Kong or Macao, see the following -- both SIM cards are inserted in the same slot, one on top and one on the bottom of the tray: Using Dual SIM with two nano-SIM cards - Apple Support

If it was originally sold elsewhere, the second SIM is an e-SIM. See: Using Dual SIM with an eSIM - Apple Support