I just learned of the Abhidharma. What was their original or early language they were written in, and do they exist online in copy/pastable (non-PDF or image) format anywhere for free? Is this of any use?
This site is pretty good.
See also Anapanasati (Mindfulness of Breathing) Buddhadasa Bhikkhu Translated by Bhikkhu Nagasena https://www.urbandharma.org/pdf/anapanasati.pdf
By the way, there are a few different versions e.g. the Theravada Abhidhamma, Sarvastivada Abhidharma, Dharmaguptaka Abhidharma etc.
Here I focus only on the most popular version, which is the Theravada Abhidhamma.
If you read the ATI page on Abhidhamma Pitaka, it says:
The Abhidhamma Pitaka has a well-deserved reputation for being dense and difficult reading. The best way to begin studying Abhidhamma is not to dive right into its two key books (Dhammasangani and Patthana), but to explore some of the more modern — and readable — commentarial texts.
The page continues to provide a list of commentarial texts, some including links.