Quasi-cyclic codes form one of the most important classes of error-correcting codes, combining strong algebraic structure with practical efficiency. They extend the idea of cyclic codes by allowing codewords to be shifted not by a single position, but by blocks of fixed length—an approach that preserves structure while significantly expanding design flexibility. This block-shift property makes QC codes attractive for both theoretical study and real-world applications, as they often achieve excellent error-correcting performance with relatively simple encoding and, in some cases, simple decoding algorithms too. As a result, quasi-cyclic codes play a central role in modern coding theory and continue to be widely used in communication systems, data storage, and even emerging fields like post-quantum cryptography. In this series of lectures, we will focus on the algebraic structure of quasi-cyclic codes, particularly from the perspective of the Chinese Remainder Theorem (CRT) and their description in terms of polynomial matrices over polynomial rings. We conclude this series of lectures with some design of quasi-cyclic low-density parity-check codes using combinatorial objects.