Ask questions about XML encoding

fallloveqq

The header of a general XML document has the following sentence
<? xml version = "1.0" encoding = "utf-8"?>
I have questions about this:
Question 1: For a text file, the file header has two bytes of FFFE to indicate that it is UTF8 encoded. What is the use of this sentence?

Question 2: If it is a network transmission or a direct byte stream transmission, then after receiving a stream that should be an XML document, it is assumed that <? Xml version = "1.0" encoding = "xxxx"?> Exists in the stream What kind of code is this paragraph? If this paragraph is xxxx code, how do I know how to read it before I have read it? Could it be notified before transmission? This problem also applies to XML file rules for other encodings (non-UTF, such as Korean, etc.).

fallloveqq

? What is wrong format?