To create custom elements you need to use the left side toolbar where you can find 5 types of tools.
Is a tool for manipulating with the elements by dragging, selecting and moving them.
The red lines indicate the binding to neighboring elements, as well as the binding in relation to the Artboard.
Hint: Hold Shift to fix the position of the element on the horizontal or vertical axis.
Select an element, hold down Alt/Option, hover over the neighboring element to see the distance between them.
Hint: you can move the elements with the arrows on the keyboard, step - 1px.
A container that allows you to create more complex structures by nesting one element into another or combining elements.
Select the Frame tool, click anywhere on the Artboard, so that a 50*50 px square frame would be created.
You can also set the size yourself, just stretch the frame on the screen.
Select the created frame. In the right panel you will find: