Set Widget as Child of Container
If you want to group widgets and UI elements in container there are two ways how you can make them a child of a container.
You can put the elements inside the Container or you drag a connection from the upper docker of the UI elements into the bottom docker of the parent.
In the children tab of the Container you can set Align to decide their order in the parent.
Another way to change the order of the child elements is to set each Align order parameter to the oder index of each child.