Many characters can infuse their element. Catalysts and bow users can readily deal elemental damage. Polearm, Claymore, Sword users usually gain an infusion through their skills. A few characters can give team-wide infusions such as Chongyun E, Bennett C6, and now Candace Q. Some infusions cannot be overridden (Xiao, Itto, Raiden, etc.). But others can be.
Certain infusion abilities will constantly apply or 'refresh' the infusion to the weapon (Chongyun, Bennett, Diluc). Let's call these strong infusions. But other infusion abilities will not refresh (Keqing E, Kazuha C6, Ayaka after sprinting). Let's call these weak infusions. Because weak infusions do not refresh, they can be overridden by the stronger infusions. For example, C6 Kazuha's infusion is weak and can be consumed by Chongyun's and Bennett's strong infusion field. This consumption process actually produces a swirl reaction (based on Chongyun/Bennett's EM). So the mechanics behind infusions are related to the gauge theory.
But what happens if you have multiple strong infusions at the same time? It has been observed that when using Chongyun and Bennett's infusions at the same time, the infusion can actually oscillate between cryo and pyro.
The Bennett / Chongyun infusions can be interesting when paired with Xianyun's Q jump buff. This can give access to plunge vapes/melts. One example team is Xiao/Diluc-Chongyun-XY-Bennett. One can add Focalors here though plunging can shatter frozen targets.
For many melee characters, gaining an infusion means entering an 'enhanced' state where their damage generally increases (through elemental damage). Some characters have different levels of enhanced states, such as Alhaitham (mirrors). Some characters may have easier access to infusing their own weapon (via sprint/AA/E) than others (via Q/energy).
Examples:
It is an interesting design choice that Diluc/Noelle can keep infusion even while off-field. It is similar to how Noctis' droplets remain on-field even when switching out.
This kind of design would help characters like Cyno for hyperbloom. Cyno is locked on-field for 18s. Being able to switch off and keep the infusion and switch back in would help with flexible rotations (say if one needs to reapply dendro/Nahida's mark after the first wave is defeated). Being locked on-field means his best supports would have to be Nahida and Furina for long off-field E (and to another extents Xianyun for 16s Q vv).