There are always explanations behind the set phrases, but it is not everyone’s interest to learn the etymology of everything they learn if their goal is just to converse in the language.
Some want to go deeper and learn the story behind the phrases, and that’s fine, but I think a synchronic answer as the one above is also valid and a real answer, beside a diachronic one. You can ask us what the literal meaning or the etymology is if you want to know, but it’s not 100% sure that we know ourselves everytime.
Helst is the superlative form of gärna and it’s hard to translate to English but maybe something like ’most preferrably’. It is also found in the nowadays somewhat uncommon compound words like närhelst and vadhelst as in ”du kan komma närhelst du vill” (roughly: ’you can come whenever you prefer’) or in expressions like the ones I mentioned above as ’när som helst’ or ’vad som helst’.
I’m not entirely sure how it developed to be used in these phrases however.
I also don't know the origin of these phrases. As a native speaker, my feeling is just that they are units, I don't really feel there's anything more to them.
(compare to expressions like en björntjänst ['a disservice', literally 'a bear's favor'], where I have a clear idea why it's like that)
As Lundgren8 said, in some cases helst has historically had the meaning ever as in whenever or whoever in English, so if I were to translate it as literally as possible, maybe I'd just say they're like whoever in English, only we write them apart and add an 'as' in between.
Hints are given for all contexts, so not all hints work everywhere. You need to translate vem som helst as just anybody when there is a negation, otherwise not. There's a longer explanation somewhere else that I can't find right now, but here's one example: https://www.duolingo.com/comment/6192255
vem som helst has a meaning more like 'whoever', 'whosoever', it's a wider category which is open to "each and every" one.
någon is more specific. It can translate into English as either somebody or anybody depending on context, because we don't have the same rules for how they are distributed.
Vem som helst får komma – Anyone may come, Just anyone may come, Whoever may come
Någon kan komma – Someone might come
Kommer någon att komma? – Will anyone come? Will someone come?
Could you please elaborate on the different connotations of vem som helst and någon? I'm not 100% sure how to identify when or in what situations/circumstances I should use one or the other.
Is one more common than the other, or is one more formal? Are there particular situations where you would only use one and not the other? Or are they fairly interchangeable (in some cases)? I don't think I'm articulating my question very well, I just want to understand when it is and is not appropriate or natural-sounding to use vem som helst and någon I guess?
(I'm always worried about sounding like one of those people who think synonym entries in a thesaurus are all interchangeable when they're not and have distinct connotations that change the meaning or the sense of the sentence in question and stuff like that. I just want to make sure I understand the nuances of the synonyms I come across.)
It seems like som helst can be added to different words, and the whole constuction then means "anyone", "anywhere", "any time" and so on...
I'm still getting used to it. This message may not be adding much to the discussion, but it can help me to reinforce it in my head :-)