Non-orthogonal multiple access (NOMA) is a relevant technology for realizing the primary goals of next-generation wireless networks, such as high connectivity and stability. This paper focuses on the foundational principles of Power Domain NOMA (PD-NOMA), a promising technique for meeting the needs of fifth Generation (5G) in real-world applications. PD-NOMA offers flexibility in allocating radio resources to enhance user access performance. By enabling several users to utilize the same radio resources, PD-NOMA achieves improved spectrum efficiency. This paper considers the practical aspects of PD-NOMA system design, exploring various network scenarios. This paper primarily focuses on PD-NOMA that uses the superposition coding (SC) at the transmitter side in addition to the successive interference cancellation (SIC) at the receiver side. The paper also provides a theoretical formulation and solutions for downlink PD-NOMA and addresses potential areas for future research. Finally, the paper concludes by summarizing the key findings and implications of the study.