Various AoE abilities scale with the number of enemies present.
Normally, AoE attacks have linear scaling. If the attack does \(A\) damage, and there are \(n\) enemies, then the total damage is \(An\). For swirl the damage can go up to \(2An\) (max of 2 instances of damage per enemy). For Childe's riptide, the scaling is quadratic \(An^2\). For Beidou it is sublinear (scaling with \(A/n\)). In fact for most single target abilities (like Xingqiu's rainswords or Hu Tao's charged attacks), the scaling is sublinear (scaling with \(A/n\)).
Things like Albedo's Q and Ganyu's Q are bounded above by a linear function. Ganyu's Q spawns at most ~50 icicles. If each icicle hits n enemies there are 50n instances of damage, which is linear. It only appears quadratic \(10n^2\) for \(n \le 5\) but for more than 5 enemies, it is no longer quadratic but linear, being bounded above by \(50n\). Ayato's Q and Heizou's Q are similar, being bounded above by a linear function. People say these abilities have 'pseudo-quadratic' scaling. They are no longer quadratic if there are too many enemies.
By contrast, Childe has actual quadratic scaling. However large n is, Childe's riptide damage will scale with \(n^2\). This cannot be bounded above by any linear function. And that's why Ganyu and Ayato are inferior to Childe. Ganyu and Ayato are fake, whereas Childe is real.
So technically, only Childe has true quadratic scaling. But this may change with future characters.
The Anemo swirl reaction is a very broken reaction already with good aoe. One way it can be made more broken is if it had quadratic scaling.
Another thing Genshin does is putting a tradeoff between AoE radius and scaling type. Abilities with quadratic scaling (or pseudo-quadratic scaling) typically have smaller AoE radii (like within 1-3m), requiring enemies to be grouped up very closely together. Typically this is achievable with vacuum abilities from Venti, Kazuha, etc. which is why Anemo units are extremely valuable. On the other hand, many abilities with linear scaling (such as Ganyu's CA, Xiangling's Q, or Xiao's plunging) have much larger AoE radii (5-6m or more). Moreover the damage talent scaling is higher for linear abilities (expect 200-400% or more), whereas for the quadratic abilities, the damage scaling is a bit less (riptide damage is like around 100%, etc.) But this may change with future character abilities.
So the game is basically telling you: sure you can have quadratic scaling but the AoE radius will be smaller, or you can have the typical linear scaling but the AoE radius can be bigger. This is how the game balances AoE mechanics.
Most abilities scale linearly with a stat such as atk, hp, def, er, em, etc. There is a character that scales quadratically with a stat: Alhaitham with EM. There is also a character that scales cubically with a stat: Nahida with EM.
This means that the character's damage scales with that power of the stat. So Alhaitham's damage scales like \(O(EM^2) \).