This study explores how peer learning and teaching interactions in undergraduate STEM courses occurred through the "Peer-Learning Assistant" (PLA) model. This is one of the active learning strategies applied in a curricular setting, and also to find out the learning experiences for STEM students and PLAs from the PLA model. For this study, a qualitative method approach was applied, specifically a case study using the PLA model in a research-oriented university in the Southern region of the United States. Specifically, four STEM classes were observed, and 6 PLA students and 11 students who were served by the PLAs were interviewed. The findings reveal the characteristics of interactions between PLAs and students through PLA model varied by class design and style; however, PLAs and students achieved learning improvement through the same PLA model. The benefits of the PLA model and what should be performed for the model development are discussed.