The COVID-19 pandemic shifted higher education online, drawing attention to synchronous learning and instruction on digital communication platforms. Learner-centered teaching practices in the tertiary level, such as mathematical discussions, have been shown to benefit student learning. The interactions involved in online synchronous mathematical discussions have been studied less. Most research taps into these aspects drawing on students' academic outcomes and reflective interviews. This study explores instructional practices as they unfold online, with a focus on student-instructor and student-student interactions. We zoom-in to online synchronous teaching and learning processes on a popular communication platform by analyzing Linear Algebra tutorials in the first pandemic year. Using the commognitive framework, we characterize instructional interactions with the construct of a learning-teaching agreement. The analysis resulted in three interactional patterns, where in all cases, the tutorial transformed at some point to the one dominated by the instructor.