When 0x12345678 is stored in little endian, it looks like 78 56 34 12 in a byte dump, which is stupid because hex digits are grouped as pairs and then revered.

The endianness at the bit level is irrelevant because the bits are chunked into bytes, and are usually not even addressable.

A storage format exhibits endianness only when it is addressable.

In the C language, bits are only "addressable" via the shift operators. These are rooted in pure arithmetic, so that 1<<1 is always 2, regardless of whether you're on a big or little endian platform. Whether you call the value 1 "bit 7", "bit 8", "bit 1" or "bit 0" is just, pardon the pun, word semantics.

The only time you deal with "bit endianness" is with certain compressed data formats, and with bitfield layout rules.