Elemental Types in Honkai Star Rail
A Xianzhou-appropriate explanatiom for Star Rail's in-game elemental damage system would probably be the concept of "Spiritual Roots". It's the standard ENG term for [elemental affinity] in chinese fantasy fiction.
(this is largely unrelated to the naming scheme of lunar years by classical elements + zodiac animals: someone born in the year of the Wood Ox does not necessarily have a "Wood" spiritual root)
Unlike in western fantasy, chinese fantasy favours having one "pure" spiritual root as opposed to having multiple [diluted] elemental affinities. The logistics vary depending on the setting but, generally speaking, a person with one root would be stronger than a person with multiple roots.
(those familiar with Naruto may find this concept familiar: it's the same system)
The "classical" elements in chinese fantasy fiction come from Wuxing philosophy: "water", "wood", "fire", "earth" and "metal". Popular additions include "wind", "ice" and "lightning". It is thereby plausible to assign Jing Yuan a "lightning" root and Jingliu a "mutated water"/"ice" root".
It is inadvisable, however, to assign a spiritual root of "darkness" or "light" to a character. That's because they philosophically overlap with the concepts of "yin" and "yang". All beings requires both energies to exist in balance, nevermind any gendered binaries attributed to them, as it is only together that these energies form [life energy] (qi).
Where the Hoyoverse is concerned, "imaginary" is vaguely analagous to "yang", both share connotations of [life], [light], [warmth], and [creation]. More specifically, the term "imaginary" references the concept of the "Imaginary Tree". It is described as a kind of World Tree of World Trees, one encompassing every universe and every timeline, with travel between worlds faciliatated by crossing the Tree's branches.
"Yin" is, accordingly, made equivalent to "quantum" energy and they share connotations of [death], [darkness], [coldness], and [entropy]. This is in reference to the "Sea of Quanta": the void-waters that feed and eat away at the Imaginary Tree.
Yin and yang are not explicitly gendered but the hets love to restrict things into binaries so there's biases against yin-coded men and yang-coded women (not that this stops mihoyo from invoking gender role subversions at every opportunity).
Yes: this IS why there aren't any quantum type dudes in Star Rail yet.
The restriction of Star Rail characters to specific elements is more of a gameplay restriction than an in-universe one: the elements assigned to characters are largely based on aesthetic and balancing purposes.
Dan Heng, for instance, is intended to be aligned with the Wood element, as befitting the High Elder of the XZ Luofu (home of the Ambrosial Arbor). His name and aesthetic points to this affinity but, since there's no "wood" element in the combat system, he was given the "wind" element instead (which could just be a pun on his past incarnation's name tbh). "Imaginary" is more thematically appropriate for him but even that is an accomodation for the lack of a "water" element.
Technically speaking, Blade should be a "physical" damage dealer: he was probably assigned "wind" for aesthetic purposes and to better match Dan Heng.
Jingliu is icy for the same reasons that Genshin's Skirk is "cryo": they are yin-coded moon maidens loosely based on the human form of HI3's Benares, the Herrscher of the Void's pet space dragon. Jingliu's leaning towards "ice" over "quanta" seems to be an aesthetic thing and a means to match her with her grand-disciple, Yanqing.
This post is by no means exhaustive where elemental gameplay versus in-game lore is concerned but it will hopefully serve as a primer to those less familiar with east asian fantasy staples. Feel free to ask questions or make ammendments! ^_^
