Which new report template should you use when you want to create a net-new report to add to the left-hand nav that details source / medium filtered to desktop users only?

Use the Traffic Acquisition template because this report is specifically designed to analyze how users arrive at your site, including dimensions like source and medium, which aligns directly with the requirement to detail source/medium performance. By starting with the Traffic Acquisition template and applying a filter for desktop users, you can create a customized report that meets the criteria and add it to the left-hand navigation, whereas the other templates focus on landing pages, demographics, or technical attributes rather than traffic source analysis.