4 ways allow you to create complex objects in Spline:
Use groups (cmd+G / ctrl+G) to combine objects together. Every object inside the group will save its default options. Keeping several objects in one group allows you to move/scale/rotate objects together.
Every group has its pivot point in the centre. You can change the pivot point's position by clicking on the group icon (keep pressed opt+cmd / alt+ctrl).
Boolean Modifier
You can create an object by using one of the boolean modifiers. There are three options for the boolean modifier:
Remember, to create a shape using the boolean modifier, you need at least two objects from the following: