You can add any element to another one with .appendChild( childElement ). So if you want to add the "li" to the body of your document you can do this:
document.body.appendChild( li )
So, in order to achieve what you asked, you should make this:
function addUserDataToDom(data) {
const li = document.createElement('li')
const userList = document.getElementById('user-list')
const userName = data.items[0].login
const id = data.items[0].id
const avatarUrl = data.items[0].avatar_url
// Create p elements to add your user data
const pUserName = document.createElement("p")
const pId = document.createElement("p")
const pAvatrUrl = document.createElement("p")
pUserName.innerText = userName
pId.innerText = id
pAvatrUrl.innerText = avatarUrl
li.appendChild( pUserName )
li.appendChild( pId )
li.appendChild( pAvatrUrl )
}
As you can see, all the data was added as innerText inside the p element.
Finally you have to append those p elements we created earlier inside your li element.