Explain the internal working of the D-Bus wire protocol message format?

Every D-Bus message on the wire consists of a fixed-layout header followed by a body, both marshaled according to the type system and alignment rules the specification defines, so any conforming implementation can parse a message from any other without prior coordination.

The header starts with a single byte indicating endianness (little or big), followed by the message type (method call, method return, error, or signal), a set of flags such as NO_REPLY_EXPECTED, the protocol version, and the body's length in bytes. Next comes the serial number, then a variable-length array of header fields, each an (integer code, variant value) pair encoding things like the object path, interface, member name, destination, sender, and, for replies, the reply_serial.

After the header is padded out to an 8-byte boundary, the body follows, containing the actual method arguments or signal payload marshaled strictly according to the type signature declared in the header's SIGNATURE field, using the same alignment rules applied throughout, which is what allows a receiver to walk the body and extract each argument without ambiguity.