The difference between scattering and reflection is that the direction of the former is unpredictable and the direction of the latter is predictable.
Scattering is divided into Rayleigh scattering, Michaelis scattering and non-selective scattering, and the corresponding object size is much smaller than the wavelength, close to the wavelength and slightly larger than 10 times of the wavelength.
Reflection can be divided into ideal specular reflection, basic ideal specular reflection, basic ideal diffuse reflection and ideal diffuse reflection.
Therefore, "optical sensors mainly obtain reflectivity information of ground objects, and microwave sensors mainly obtain scattering coefficient information" is actually a very rough sentence. A rough explanation is that light waves and microwaves are electromagnetic waves with different frequencies and wavelengths. If the size of ground objects is equivalent to the wavelength of microwave signals (300MHz~300GHz), ranging from a few millimeters to 1 meter, then the wavelength of visible light (400 ~ 700 nm) is much larger. The general idea is to judge the reflection and scattering characteristics according to the wavelength and the size of the object. See [1] for a more detailed explanation.
References:
[1]John R. Jensen, environmental remote sensing: a view of earth resources, Prentice Hall, 2000.