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.