Some machines require additional machine cores to function. These must be placed at specific positions relative to the machine itself. The number of cores needed for a machine is listed in the machine's tooltip. Cores cannot be shared between machines.
To begin the multiblock construction, first place down the machine itself.
When right-clicking the machine, all blocks that need to be cores will be highlighted for a few seconds. You can then place the cores there. Alternatively, you can right-click the machine with a core in your hand, which will automatically place the next core. Note that this only works if the target locations are unobstructed. If there is an obstacle, the block will be highlighted instead.
Each machine core has a quality associated with it. Using better machine cores increases the total quality of the machine.
To view the resulting quality of a machine, hover over the machine icon in the top left of the UI. The machine quality itself does not directly affect machine operation. Instead, it determines how many addons can be added to a machine. If you don't plan on adding addons to the machine, you can use the cheapest cores.