There are few basic types in packets used. Most of them are classic C types, but there are few small differences.
This type is made of uint16, which specifies in bytes how long the string is and followed by the text itself.
This is sequence of bytes.
Other types are classical C types, but they are just missing _t suffix. Also there is usually prefix u, that means those values are unsigned. But this prefix can be often wrong as it is hard to correctly determine.