How std::transform and std::plus work together?

I was reading C++ reference and came across std::plus function with an example. Which is pretty straight forward, its simply adds lhs and rhs. The code was:

#include <functional>
#include <iostream>

int main()
{
   std::string a = "Hello ";
   const char* b = "world";
   std::cout << std::plus<>{}(a, b) << '\n';
}

output: Hello world

I changed it to

#include <functional>
#include <iostream>

int main()
{
   int a = 5;
   int b = 1;
   std::cout << std::plus<int>{}(a, b) << '\n';
}

output : 6

Now I made

foo vector = 10 20 30 40 50
bar vector = 11 21 31 41 51

I called:

std::transform (foo.begin(), foo.end(), bar.begin(), foo.begin(), std::plus<int>());

and it gave 21 41 61 81 101 which I understand it is adding up both foo and bar. But how it was passed to std::plus function?

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