append()
accepts either a node or a string value. This you might want to use if you want to append some text as a child.
appendChild()
accepts only a node, so you would need to create a DOM element before using it.
const container = document.getElementById('container');
const child1 = document.createElement('b');
child1.append('Bold');
const child2 = document.createElement('u');
child2.append('Underline');
container.append('Normal!');
container.append(child1);
container.appendChild(child2);
container.appendChild('And more');