Instructors get pretty good at monitoring the questions posted by students while they are lecturing. If the teacher missed seeing a posted question, there is a hand raise tool.
Typically, instructors allow text chat among students so long as it is on topic. If the chat begins to stray off-topic among students then they will typically remind students to focus.
Even when chat is on topic, some students are easily distracted by the sensory changes of the updating chat window. For these students, they can minimize the chat box.
In some classes, teachers will allow video and audio during class. Whether this is done depends on class topic as well as class size. In very large classes, the classroom drags if everyone is on audio/video. Most teacher give a couple students access at a time so there is not too much background noise from the homes. Little siblings talking or dogs barking from multiple student microphones would be distracting during class.