Explain how to send bitcoins from multiple addresses to one
How would he do it?


It depends on the wallet manager you are using. 

If you mean how does it do it under the hood it adds multiple inputs to the transaction and signs it with the private key of each address.


  What about miner fee


Considering that the suggested fee should be proportional to the weight in terms of data of the transaction, since a transaction from multiple addresses should be smaller (always in terms of data) of multiple separate transaction the fee can be slightly smaller too.

Yust to give you an idea writing

A -&gt; D
B -&gt; D
C -&gt; D


Takes more space than writing

A, B, C -&gt; D