I think bro didn't watch the show. Avatars can theoretically learn sub-bending techniques from all four elements, but being the Avatar doesn't automatically make someone capable of mastering every one. Korra and Kyoshi are good examples of Avatars who demonstrated rare bending abilities.
Avatar Korra learnt Metalbending during The Legend of Korra, becoming one of the few Avatars shown using the technique (bending metal).
Avatar Kyoshi — Lavabending: She demonstrated the ability while separating the Kyoshi Peninsula from the mainland.
Avatar Roku — Lavabending: He manipulated lava during the volcanic eruption at his home.
Avatar Szeto — Lavabending: In an Avatar State flashback in The Last Airbender, he manipulated volcanic activity across multiple volcanoes.
Avatar Aang — Energybending: He learned energybending from a lion turtle and used it to remove Ozai's firebending. This is a separate discipline, rather than a sub-element of the four traditional elements.
The important distinction is that some abilities require specific talent, while others can be learned through training. Metalbending, for example, was considered impossible until Toph discovered how to do it. Lavabending is much rarer, and its exact limitations remain unclear.
Is there any specific reason why we’ve never seen an Avatar who can bend every single sub element across all four elements? Or is it just not possible for an Avatar to master all of them?