Micro.blog uses Hugo-- all the customizations you’re discussing in various threads require and understanding of Hugo and its templates and very little Micro.blog-specific knowledge. Here are the Hugo docs.
In this case, your RSS template is probably not included in your theme, but you can customize it when you click to customize your theme by scrolling down to the default templates, which are included if you do not override them from Micro.blog. There, layouts/_default/rss.xml is what builds your RSS feed.