There is Array.push function in solidity how it is return length of the array?

Question

pragma solidity >=0.5.0 <0.6.0;

contract ZombieFactory {

event NewZombie(uint zombieId, string name, uint dna);

uint dnaDigits = 16;
uint dnaModulus = 10 ** dnaDigits;

struct Zombie {
    string name;
    uint dna;
}

Zombie[] public zombies;

function _createZombie(string memory _name, uint _dna) private {
    uint id = zombies.push(Zombie(_name, _dna)) - 1; //**here how it is return length of array**
    emit NewZombie(id, _name, _dna);
}

function _generateRandomDna(string memory _str) private view returns (uint) {
    uint rand = uint(keccak256(abi.encodePacked(_str)));
    return rand % dnaModulus;
}

function createRandomZombie(string memory _name) public {
    uint randDna = _generateRandomDna(_name);
    _createZombie(_name, randDna);
}

}

I want to know how it is working i tired to searched in google but i cant get exact results try to explain how it returning length of array and basically it just add the element ...

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