It depends. For me it's a 20% role, I focus more on the sprint flow. We have a System Engineer and they do most of the Product Owner cat wrangling that I would assume a lot of Scrum Master's have to handle.

The scrum team should be acting as a team and the developer should work with the team to figure out how he aligns. That is part of the "Self Forming" stuff. The Scrum Master is not their boss and does not weigh in on the project implementation. They are more of the advocate for the team and the advocate for Agile/Scrum. e.g. Dealing with the Product Owner when they try to add stuff to the current sprint or Management saying everything is top priority. Technically we call that coaching the Scrum process, but I'm going to make sure my team has steady/predictable work and are not going to burn out.