Yeast Single-Cell Proteins (SCP) production using various agro-industrial byproducts and wastes have significant potential as an alternative to the soy meal, and fish meal protein used for livestock and aquaculture feeds. The use of organic wastes as a substrate in the fermentation processes can be accepted as one of the solutions to reduce the total price of the culture and an environmentally friendlier method of removing these residues. This review article focuses on the yeast biomass yield and protein content increase strategies, which is impossible without understanding metabolic pathways and switching mechanisms. The present work discusses optimization strategies for protein-enriched yeast biomass production, such as fermentation medium composition, including a selection of carbon and nitrogen sources and their ratio, supplemented trace elements, and cultivation conditions such as pH, temperature, time of cultivation, and inoculum size. This review summarizes the theoretical knowledge and experimental results of other researchers that provide an overview of the achievements of the last decades in the production of SCP.