The title of the Metaphysics (τὰ μετὰ τὰ φυσικά) is not Aristotle's. He describes his inquiry as "theology" and "first philosophy."
This description raises a question about whether his inquiry in the Metaphysics is into more than one subject. First philosophy is the study of "being as being." Theology is a study of divine objects. So theology and first philosophy would appear to to be different subjects. Theology is a study of some of what exists, the divine objects. First philosophy studies existence in general.
The problem for the historian is to determine what if anything Aristotle thinks unifies theology and first philosophy as a single subject.
What Aristotle has in mind is not completely clear, but his idea seems to be that there are "ways of being." He seems to think that the way of being or existence that belongs to divine objects is first among the ways. Divine objects exemplify being most of all. In this way, because the being of divine objects is most of all what being itself is, and because all other ways of being are qualifications of the way of divine objects, theology is first philosophy: it provides the explanation not only of primary and divine being but of being and existence in general.
One might wonder whether the primary philosophy (πρώτη φιλοσοφία) is universal or deals with some one genus or entity..... If there is not some other substance (οὐσία) besides those which are naturally composed, physics will be the primary science; but if there is a substance which is immutable (ἀκίνητος), the science which studies this will be prior to physics, and will be primary philosophy, and universal in this way, that it is primary. It will be the province of this science to study being as being; what it is, and what the attributes are which belong to it as being" (Metaphysics VI.1.1026a).
The Search for Substance
"Indeed long ago, now, and always, what is sought after and always puzzled over, what is being (τί τὸ ὄν), is the question what is substance (οὐσία)" (Metaphysics VII.1028b).
The noun οὐσία derives from the feminine present participle of εἰμί (“I am, I exist”). The infinitive form of εἰμί is εἶναι ("to be" or "being"). The singular nominative/vocative present participles of εἰμί are ὤν (masculine), οὖσα (feminine), ὄν (neuter). The word οὐσία is often translated into Latin as substantia and into English as substance. This translation reflects the idea that things in other categories of existence depend for their being on the substances. The Latin substantia more literally translates the Greek word ὑπόστασις, which is a compound of ὑπό ("under") and ἵστημι ("I stand").
"Traditionally οὐσία has been rendered by 'substance.' The reason for this is that, on the view Aristotle puts forward in the Categories, properties depend for their being on objects in that objects are their ultimate subjects, they are what ultimately underlies everything else. Indeed, objects in the Categories are characterized by the very fact that they are the ultimate subjects which underlie everything, whereas there is nothing that underlies them as their subject. It is because of this characterization that the rendering 'substance' seems appropriate" (Michael Frede, "Substance in Aristotle's Metaphysics," 73).
Metaphysics VII.1028b suggests that Aristotle understands work in ontology in the Presocratics and in Plato in terms of the "search for substance." So, for example, as Aristotle seems to understand him, Democritus thought that atoms and void are the substances and that everything else that exists, exists in terms of these substances.
(Note that the way Aristotle seems to understand Democritus is not the way I set out his view in the unit on the Presocratics. Democritus and Leucippus, as I understood them, thought that nothing exists but atoms and void. Atoms and void can stand in various transitory arrangements, but these arrangements do not form objects. When the arrangements cease to hold, nothing has gone out of existence. Similarly, nothing has come into existence when they begin to hold. Coming into and going out of existence is how certain changes in what there is appear to human beings.)
Aristotle is both Platonist and Platonic critic. He thinks that forms are substances, but he thinks that Plato was wrong about the way in which forms are substances.
Aristotle thinks that forms in matter are the substances of natural bodies. The form in matter is the reality of the natural body, but the substances of natural bodies are not substances most of all. Forms in matter are substances, but they are substances with qualification. Aristotle thinks that there is a more basic and fundamental reality. This reality is substance most of all. It is substance without qualification. The way of being of this substance is primary. This way of being is divine being. The other ways being or existence are understood in terms of this being.
Aristotle's view seems to have been a work in progress
This view on substance in the Metaphysics seems to have been something that Aristotle worked out over time and never set out in complete detail. Further, the Metaphysics itself is not easy to read. It is one of the most difficult works in ancient philosophy and one whose interpretation remains a matter of considerable dispute among historians. This makes Aristotle's view hard to know, but it is possible to get some insight into what Aristotle was thinking by seeing how his views changed from the Categories to the Metaphysics. The picture that emerges suggests that over time Aristotle becomes more sympathetic to broad framework of Platonism. He starts off as a Platonic critic in the Categories, but later in the Metaphysics he reinterprets Platonism and its commitment to forms.