Here's a couple facts about Avatar: The Last Airbender that I couldn't fit in my video.

We already know the series has anime references, and they were largely inspired by series that had become popular in the late 90s and early 2000s. https://twitter.com/CanipaShow/status/1290725440852840449

For example, King Bumi, with his lanky frame and long arms was directly based on the proportions of an Evangelion unit.

Aang's glowing tattoos in his Avatar State were inspired by Naota's "final form" in FLCL

Jet's design was based on Spike from Cowboy Bebop

In a more general sense, the entire team was fans of the works of GAINAX and Ghibli and so they sought to find a Japanese studio to create Avatar. Failing that, they looked towards Korea. The team still had a load of anime experience though.

Between the pilot and the full series of Avatar, JM Animation worked heavily on both Aquarion and Gankutsuou: The Count of Monte Cristo. Although they were heavily involved in Avatar, Shoji Kawamori was one of their top clients.

So when Art Director Bryan Konietzko came to Korea and talked about doing something with anime-style action, the animators were already more than qualified to work in that style, leading some to believe that it was created in Japan.