Short answers, pulled from the story.
Abhidharma literally means "about the dharma." The prefix abhi can mean "concerning" or "about," and this was its earliest sense in texts like the Mahagosingha-sutta. A later interpretation read abhi as "higher" or "superior," giving the meaning "higher teaching," but scholars regard this as a secondary development.
Modern scholars generally date the earliest canonical Abhidharma texts to around the 3rd century BCE, after the time of Gautama Buddha. Erich Frauwallner placed the main period of canonical Abhidharma development between 250 and 50 BCE.
Matikas (Sanskrit: matrikas) are early Buddhist doctrinal lists that scholars including André Migot, Erich Frauwallner, and Rupert Gethin identify as the ancient core from which Abhidharma developed. They served as memory aids for teaching and preserving doctrine, and the Abhidharma tradition grew as monks added commentary to these lists and combined them into more comprehensive frameworks.
Both traditions have a canonical collection of seven books, but the texts differ substantially. The Sarvastivada held that dharmas exist in all three times (past, present, future), while the Theravada and related schools accepted only the present moment as real. The Theravada enumerated 82 types of dhammas; the Sarvastivada enumerated 75. These differences are more pronounced between the two Abhidharma collections than between their respective sutra collections.
The Abhidharmakoshabhashya, known as the Treasury of Higher Knowledge, is a 5th-century work by Vasubandhu consisting of verses and commentary. It often critiques Vaibhashika views from a Sautrantika perspective. It remains the primary source for Abhidharma study in both Indo-Tibetan and East Asian Buddhism.
Buddhaghosa was a South Indian exegete who moved to Sri Lanka and wrote in Pali during the 5th century CE. His Visuddhimagga, or Path of Purification, is a comprehensive manual of Buddhist practice that includes an overview of the Abhidhamma. It remains the central reference work of the Theravada school.