The assembly of secondary structural units into larger functional units such as the mature 
polypeptide and its component domains is
The modular nature of protein synthesis and folding are embodied in the concept of orders of protein structure: 

Primary structure—the sequence of amino acids in a polypeptide chain
Secondary structure—the folding of short (3-30 residue), contiguous segments of polypeptide into geometrically ordered units.
Tertiary structure—the assembly of secondary structural units into larger functional units such as the mature polypeptide and its component domains.
Quaternary structure—the number and types of polypeptide units of oligomeric proteins and their spatial arrangement.

Ref: Harper’s illustrated biochemistry. 30th edition page no: 37
Primary structure
Secondary structure
Tertiary structure
Quaternary structure