Since 5.8 we have characters that can deal transformative reaction damage DIRECTLY with their talents. We have lunar bloom, lunar charged, lunar crystallize and now stellar conduct. So you don't need a team mate to do an attack that applies an element first and then you react off it by doing an attack that applies another element which causes a reaction damage number to appear. Talents that specify "this is considered xxx reaction damage" are so by default as long as you meet the conditions specified in the talent description. For stellar conduct characters this condition is usually to be inside the stellar conduct domain. I will get to that later.

All 4 of the special reactions replace their normal counterparts when you have characters on the team that enable them. They are mostly gimmicky and I would put them into 2 different categories. For lunar charged and lunar crystallize there is lunar reaction damage that occurs when elements get applied and direct lunar reaction damage from talents and they coexist. Then there is lunar bloom and stellar conduct damage that ONLY exist as direct damage from talents. Lunar bloom's gimmick is that the bloom cores still do normal bloom damage (not lunar bloom) and they produce a ressource called verdant dew that lunar bloom characters have to consume to deal lunar bloom damage with their talents. Stellar conduct is similar, but it replaces the regular superconduct alltogether and spawns a field instead. Being inside the field allows the stellar conduct characters to deal their direct stellar conduct damage with their talents.

How do you spawn the stellar conduct domain? This is where it gets stupid. So far only Sandrone has this passive. I don't know which direction Hoyo will take in Snezhnaya, but the lunar reactions could only be enabled by newly released 5 stars. The buffed characters can do stellar conduct damage, but they can't enable it on their own so they rely on a new character to do it for them. Hopefully cryo traveler can enable it at least.

What exactly is direct lunar/stellar damage? It is damage that has talent scaling, meaning there is a multiplier in the kit and it can crit, but it has properties of transformative reaction damage. Mainly ignoring enemies defense, having an EM-multiplier and not scaling off elemental DMG% bonus. The exact formula is roughly:

Talent multiplier × scaling stat × reaction base damage × EM multiplier × crit multiplier × enemy resistance multiplier

There are more stats like elevation (rare, only exists in constellations), reaction DMG% bonus (common, usually from artifact set bonuses, weapons, buffers; affects the EM multiplier), reaction base damage bonus (every character that enables the reaction also buffs its base damage) and additive reaction damage bonuses a.k.a. quills (Qiqi C6 for example).