Where does one get basic notions of Buddhism when starting with Mahayana? Maybe it's better to use an example. I believe one of the very first Mahayana sutras one is expected to learn is the Heart Sutra. But when you read it, you immediately see things like 'skandhas' and 'dharmas'. If you start with Pali Canon, you can basically find all about them in the Sutta Pitaka. But if you know nothing about Theravada, where in the Mahayana teachings do you find those basic notions? What will tell you about karma, dependent arising, dharmas, worlds structure and so on?
In Mahayana these kinds of things are learned from the encyclopedic compendiums summarizing the basic concepts by categories, known as "Mahayana Abhidharma" and its predecessor "Sarvastivada Abhidharma".

The main idea to understand here is how Mahayana emphasizes passing-on the intended meaning behind the scripture. So even though the original sutras were available in Mahayana in form of Agamas, the schools almost always preferred to study various summaries trying to organize the Dharma and distill it to its key principles. Lineage-specific commentaries then were used to "unpack" the meaning stored in the summaries and present it in form comprehensible to a contemporary student.

The tradition even has a poetic image for making these summaries. It's called "churning butter from the milk of Dharma". From Mahayana viewpoint the best butter was made just around time when Yogacara school was emerging from Sarvastivada, because that's when this kind of summarizing work was "in vogue", so many best minds spent lots of effort on doing this, resulting in texts some of which are still unsurpassed even to our days.

The exact texts vary by lineage, but the most popular evergreens are:


Abhidharma-samuccaya ("Compendium of Abhidharma") by Asanga
Vasubandhu's Abhidharmakośa with Abhidharmakośa-bhāsya (the autocommentary)


Also, one of my personal favorites - not so popular but in the same genre - is an early work called Tattvasiddhi Śāstra.

In the past, there were many other summary texts, like "Śāriputra Abhidharma Śāstra", "Abhidharma-hṛdaya-sastra", "Jnanaprasthana", "Abhidharma Mahāvibhāṣā Śāstra" - but they all slowly fell out of active use, although many of them still exist in Chinese translations.

Anyway, back in the day when it was written, by the time someone would read the Heart Sutra, they would be very familiar with one or more of these works, so all the references to "Shariputra, the five skandhas are empty, and so are the six realms, and the four truths and so forth" - would make immediate sense, because they would speak exactly to the typical "bullet point" lists found in Abhidharma literature.