Don't understand generate unordered-list function and addEmployee function

Hi folks,

Could you please help? I couldn't understand how the unorderedlist functions generates the li html tags from "listitems" parameter. I got that with getJSON function we get the data from the server (checked and controlled with to possible states: resolve and reject). But after we need to generate the html and append it to our page from these to functions. BUT. I couldn't figure it out, how this 'listItems' parameter works when we call the function after ".then".

  .then(generateUnorderedList)
  .then(addEmployeesToPage)

I'm talking about these functions:

function generateUnorderedList(listItems) {
    return '<ul class="bulleted">' + listItems +  '</ul>';
}

function addEmployeesToPage(unorderedList) {
    document.getElementById('employeeList').innerHTML = unorderedList;
}

My problem is that my brain works like this way:

if I would like to call a function, let's say I call generateUnorderedList(listItems), I would write generateUnorderedList(generatListItems(employees)), because I have to pass the parameter to the function. Here we didn't use anything after "then".

I wonder if you could help, specially in this part: How the program knows from listItems parameter, that we have to use generatListItems(employees)???

function generateUnorderedList(listItems) {
    return '<ul class="bulleted">' + listItems +  '</ul>';
}