This is a big question worth a full lecture.
The following links assume you already know how optical fiber works.
Here is an article that goes into some detail on absorption and multiple types of scattering https://www.fiberoptics4sale.com/blogs/archive-posts/95048006-optical-fiber-loss-and-attenuation
For multi-mode the attenuation is greater because the fiber is larger than in single-mode fiber. Image on the bottom of this http://networking.layer-x.com/p070400-1.html
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