how does stl implement gcd code example

Example: gcd in c++

#include<iostream>
using namespace std;
long long gcd(long long a, long long b) 
{ 
    if (b == 0) 
        return a; 
    return gcd(b, a % b);  
      
} 
int main()
{
	long long a,b;
	cin>>a>>b;
	cout<<gcd(a,b);
}