Many characters are made up of a semantic and a phonetic part.
I suppose the phonetic clue does not help a Japanese or Korean learner. Am I right?
Generally yes. But I want to mention that Mandarin was also a dialect. So I'm not agree that the dialects are variations of Mandarin, I think that they are parallel. Most dialects are more ancient and closer to the ancient Chinese than Mandarin.
To answer your question: Yes, the phonetic part is valid in all the dialects, since the dialects are just a variation of Mandarin Chinese, which ancient Chinese people drew upon or in which they gave ideographic meaning to various objects and concepts in the environment. So in other words, Chinese dialects are "parallel" in a way.
Most Chinese characters have been around a lot longer than modern Mandarin, so their phonetic parts often reflect much older pronunciations to the extent that these hints are actually not quite as reliable in modern Mandarin.
Take, for instance, 工, pronounced gōng in Mandarin; it is used as a phonetic in the characters 江、紅、項, but they are now pronounced jiāng, hóng, and xiàng, respectively, even though their pronunciations were much closer in older varieties of Chinese. Cantonese, for instance, is considered a more phonologically conservative variety of Chinese that preserved more of the rimes from Middle Chinese. As such, 工、江、紅、項 in Cantonese are pronounced more similarly: gūng, gōng, hùhng, hohng (using Yale romanization).
In general, all of the Chinese varieties each have underwent their own set of sound changes. Because sound changes are typically systematic, words that were pronounced similarly tended retain their similarity even as their underlying pronunciations changed so the phonetic parts of characters are often still relevant. However, as shown by the example, sometimes initial slight differences between them could also end up being magnified over time.
Regarding your question about Korean and Japanese, while they are not related to Chinese, they did initially borrow the pronunciations that came with those characters as well when they borrowed the vocabulary from Chinese. In the case of Japanese though, they assigned their own non-Chinese readings to many characters, so any phonetic hints in the original characters would no longer apply to those readings.
The Korean language (to my knowledge at least) used Chinese characters only for Sino-Korean words of Chinese origin, and did not assign non-Chinese readings to them; for this reason, the phonetic hints for Chinese character readings in Korean are still relevant. For instance, according to Wiktionary, the pronunciations of the four example characters above in Korean are gong, gang, hong/gong, and hang.
Note that an added complication is that different words may have been imported into Korean and Japanese during differing periods, so their pronunciations will reflect the state and variety of Chinese used at that time. This may also end up making the phonetic hints not as reliable.
Furthermore, the phonetic components are even often valid in Japanese, which borrowed a huge amount of Chinese vocabulary along with the writing system. For example, 放 has the phonetic component 方 and both have the on'yomi (originally Chinese) reading ほう (hō).
Yes, all are parallell in the sense that all chinese characters contain semantic, phonetic, or ideographic components, and therefore are some combination of those three as a whole. Whether its chinese, japanese, korean, mandarin, cantonese, wu, this applies. It is part of how chinese characters inherently work and how they are designed as a concept.
That said, none of them are truly parrallell to each other in set patterns, at least not in 100% predictable ways. If you compare the patterns of mandarin to cantonese to middle chinese, you will definitely see similarities, component A makes sound B in mandarin and sound C in cantonese for example pattern.
However, maybe you also encounter some characters with the same component A, but in cantonese they are pronounced D and in mandarin they are pronounced C. So patterns yes, actual set parallells no. they aren't the same, and the way they have combined or seperated the sounds of the past have changed those patterns.
Same applies to different types of mandarin, different types of cantonese, etc etc. Obviously the smaller the difference of the types of chinese as a whole, the more parallell they will run-- but unless you go all the way to the same type those patterns won't be fully the same.
Even if you are only looking at one single thing like standard mandarin, it won't always be parallell to itself, because some chinese characters chosen to represent a definition or sound for reasons unrelated to their components inherently. I suppose you could think of those as false positives. Others have been changed very intentionally from what they originally were, which means the current components don't match the source.
So yeah, even in the same exact type of chinese, the patterns are strong but not strong enough to be a set pattern. No one would make fun of a language learner for it, but if you try to guess a character's meaning or pronunciation based on its components thats likely to follow with a joke about you being illiterate.
Hope this makes sense and helps （＾ν＾）
