(1) What does byte sequence mean, an arrary of char in C? Is UTF-16 a byte sequence, or what is it then? (2) Why does a byte sequence have nothing to do with variable length?
You seem to be misunderstanding what endian issues are. Here's a brief summary.
A 32-bit integer takes up 4 bytes. Now, we know the logical ordering of these bytes. If you have a 32-bit integer, you can get the high byte of this with the following code:
uint32_t value = 0x8100FF32;
uint8_t highByte = (uint8_t)((value >> 24) & 0xFF); //Now contains 0x81
That's all well and good. Where the problem begins is how various hardware stores and retrieves integers from memory.
In Big Endian order, a 4 byte piece of memory that you read as a 32-bit integer will be read with the first byte being the high byte:
[0][1][2][3]
In Little Endian order, a 4 byte piece of memory that you read as a 32-bit integer will be read with the first byte being the low byte:
[3][2][1][0]
If you have a pointer to a pointer to a 32-bit value, you can do this:
uint32_t value = 0x8100FF32;
uint32_t *pValue = &value;
uint8_t *pHighByte = (uint8_t*)pValue;
uint8_t highByte = pHighByte[0]; //Now contains... ?
According to C/C++, the result of this is undefined. It could be 0x81. Or it could be 0x32. Technically, it could return anything, but for real systems, it will return one or the other.
If you have a pointer to a memory address, you can read that address as a 32-bit value, a 16-bit value, or an 8-bit value. On a big endian machine, the pointer points to the high byte; on a little endian machine, the pointer points to the low byte.
Note that this is all about reading and writing to/from memory. It has nothing to do with the internal C/C++ code. The first version of the code, the one that C/C++ doesn't declare as undefined, will always work to get the high byte.
The issue is when you start reading byte streams. Such as from a file.
16-bit values have the same issues as 32-bit ones; they just have 2 bytes instead of 4. Therefore, a file could contain 16-bit values stored in big endian or little endian order.
UTF-16 is defined as a sequence of 16-bit values. Effectively, it is a uint16_t[]. Each individual code unit is a 16-bit value. Therefore, in order to properly load UTF-16, you must know what the endian-ness of the data is.
UTF-8 is defined as a sequence of 8-bit values. It is a uint8_t[]. Each individual code unit is 8-bits in size: a single byte.
Now, both UTF-16 and UTF-8 allow for multiple code units (16-bit or 8-bit values) to combine together to form a Unicode codepoint (a "character", but that's not the correct term; it is a simplification). The order of these code units that form a codepoint is dictated by the UTF-16 and UTF-8 encodings.
When processing UTF-16, you read a 16-bit value, doing whatever endian conversion is needed. Then, you detect if it is a surrogate pair; if it is, then you read another 16-bit value, combine the two, and from that, you get the Unicode codepoint value.
When processing UTF-8, you read an 8-bit value. No endian conversion is possible, since there is only one byte. If the first byte denotes a multi-byte sequence, then you read some number of bytes, as dictated by the multi-byte sequence. Each individual byte is a byte and therefore has no endian conversion. The order of these bytes in the sequence, just as the order of surrogate pairs in UTF-16, is defined by UTF-8.
So there can be no endian issues with UTF-8.
