Might be that terminology is throwing you off? I’d recommend Mao’s On Contradiction: https://www.marxists.org/reference/archive/mao/selected-works/volume-1/mswv1_17.htm

In particular, note how he frames it here:

The metaphysical or vulgar evolutionist world outlook sees things as isolated, static and one-sided. It regards all things in the universe, their forms and their species, as eternally isolated from one another and immutable. Such change as there is can only be an increase or decrease in quantity or a change of place. Moreover, the cause of such an increase or decrease or change of place is not inside things but outside them, that is, the motive force is external. Metaphysicians hold that all the different kinds of things in the universe and all their characteristics have been the same ever since they first came into being. All subsequent changes have simply been increases or decreases in quantity. They contend that a thing can only keep on repeating itself as the same kind of thing and cannot change into anything different. In their opinion, capitalist exploitation, capitalist competition, the individualist ideology of capitalist society, and so on, can all be found in ancient slave society, or even in primitive society, and will exist for ever unchanged. They ascribe the causes of social development to factors external to society, such as geography and climate. They search in an over-simplified way outside a thing for the causes of its development, and they deny the theory of materialist dialectics which holds that development arises from the contradictions inside a thing. Consequently they can explain neither the qualitative diversity of things, nor the phenomenon of one quality changing into another. In Europe, this mode of thinking existed as mechanical materialism in the 17th and 18th centuries and as vulgar evolutionism at the end of the 19th and the beginning of the 20th centuries. In China, there was the metaphysical thinking exemplified in the saying “Heaven changeth not, likewise the Tao changeth not”, [4] and it was supported by the decadent feudal ruling classes for a long time. Mechanical materialism and vulgar evolutionism, which were imported from Europe in the last hundred years, are supported by the bourgeoisie.

As opposed to the metaphysical world outlook, the world outlook of materialist dialectics holds that in order to understand the development of a thing we should study it internally and in its relations with other things; in other words, the development of things should be seen as their internal and necessary self-movement, while each thing in its movement is interrelated with and interacts on the things around it. The fundamental cause of the development of a thing is not external but internal; it lies in the contradictoriness within the thing. There is internal contradiction in every single thing, hence its motion and development. Contradictoriness within a thing is the fundamental cause of its development, while its interrelations and interactions with other things are secondary causes. Thus materialist dialectics effectively combats the theory of external causes, or of an external motive force, advanced by metaphysical mechanical materialism and vulgar evolutionism. It is evident that purely external causes can only give rise to mechanical motion, that is, to changes in scale or quantity, but cannot explain why things differ qualitatively in thousands of ways and why one thing changes into another. As a matter of fact, even mechanical motion under external force occurs through the internal contradictoriness of things. Simple growth in plants and animals, their quantitative development, is likewise chiefly the result of their internal contradictions. Similarly, social development is due chiefly not to external but to internal causes. Countries with almost the same geographical and climatic conditions display great diversity and unevenness in their development. Moreover, great social changes may take place in one and the same country although its geography and climate remain unchanged. Imperialist Russia changed into the socialist Soviet Union, and feudal Japan, which had locked its doors against the world, changed into imperialist Japan, although no change occurred in the geography and climate of either country.

Changes do take place in the geography and climate of the earth as a whole and in every part of it, but they are insignificant when compared with changes in society; geographical and climatic changes manifest themselves in terms of tens of thousands of years, while social changes manifest themselves in thousands, hundreds or tens of years, and even in a few years or months in times of revolution. According to materialist dialectics, changes in nature are due chiefly to the development of the internal contradictions in nature. Changes in society are due chiefly to the development of the internal contradictions in society, that is, the contradiction between the productive forces and the relations of production, the contradiction between classes and the contradiction between the old and the new; it is the development of these contradictions that pushes society forward and gives the impetus for the supersession of the old society by the new. Does materialist dialectics exclude external causes? Not at all. It holds that external causes are the condition of change and internal causes are the basis of change, and that external causes become operative through internal causes. In a suitable temperature an egg changes into a chicken, but no temperature can change a stone into a chicken, because each has a different basis.

This dialectical world outlook teaches us primarily how to observe and analyse the movement of opposites in different things and, on the basis of such analysis, to indicate the methods for resolving contradictions. It is therefore most important for us to understand the law of contradiction in things in a concrete way.

My rough understanding of it is that idealism posits things as “mind over matter” to put it one way. Whereas dialectical materialism would include both mind and matter in a constant interplay, the internal and external, which then also extends beyond a single person or organism to a whole society, for example.