What's the meaning of 'whereby'?
Script
The term acquisition is preferred to "learning."
Because many psycholinguists believe that no psychological theory of learning, as currently formulated, is capable of accounting for the process whereby children come to achieve a fluent control of their native language.

Question
1. What is the meaning of 'whereby' in this script?

2. The term acquisition is preferred to "learning."
- Is it mean that "acquisition" is much more popular than "learning" in terms of usage?

Thanks for your kind help!

Question

What's the meaning of 'whereby'?
Script
The term acquisition is preferred to "learning."
Because many psycholinguists believe that no psychological theory of learning, as currently formulated, is capable of accounting for the process whereby children come to achieve a fluent control of their native language.

Question
1. What is the meaning of 'whereby' in this script?

2. The term acquisition is preferred to "learning."
-> Is it mean that "acquisition" is much more popular than "learning" in terms of usage?

Thanks for your kind help!

Answer

of how

Answer

As I understand it, I think what they mean is that children "acquire", or "adapt" to their native language. The process is spontaneous and they do not need to put effort into it.

Whereas the "learning" process would probably involve the studying - one has to put effort into it.

So the first term ("acquisition") better describes the process how children "learn" (i.e. acquire) the native language. It would be more suitable to use it in this context.

Answer

Whereby can be used instead of  "through which". 
Hopefully that will help you understand the paragraph a little better.