Short answers, pulled from the story.
An original net animation is an anime that is directly released onto the Internet. This format mirrors the term original video animation, which has been used in the anime industry for straight-to-video animation since the early 1980s.
Makoto Shinkai was a pioneer of original net animation in the late 1990s. He produced his first animated short films on a home computer and distributed them on the Internet starting with Tōi Sekai (Other Worlds) in 1997.
In the early 2000s, the Japanese anime industry began broadcasting ONA web series on the Internet. Early examples of ONA series include Infinite Ryvius: Illusion from 2000 and Ajimu appeared in 2001 alongside Mahou Yuugi that same year.
ONAs have the tendency to be shorter than traditional anime titles, sometimes running only a few minutes. Full series began to emerge with lengths comparable to standard broadcast shows during the second half of the 2010s.
Full series began to be licensed exclusively for streaming services like Netflix, Amazon Prime Video, and Disney+ during the latter part of the 2010s. These platforms started acquiring rights to distribute original net animations directly to global audiences.