That’s an interesting point.
Quite a few philosophers of cosmology have made that assertion—that all is God. But not all would agree.
This line of thinking tends to occur in forms of thought that say that the human intellect can encompass anything and everything. --If the intellect can encompass anything and everything, then we can reason accurately about everything. Thus, if God is supreme above all of our conceptions, we can say that God contains all of our conceptions and thus contains everything (with all existing things being parts of God).
This is not how western philosophy developed.
The western outlook split into a few different schools. But, between the Platonists and the Aristotelians, the tradition was defined largely by inference. The inference was that God, The One, To Hen, was beyond and before all of the things in creation. It is a bit of a mystery, but containment is not the correct relation.
God is the bit of Eternity that permeates everything and allows it to continuously create itself. It is distinct from everything in creation and contains nothing in creation.
It is that to which all aspire, when the aspiration is virtue or perfection.
I don’t know a strict definition for demons. Quite a few definitions exist and are varied across cultures. But one mechanism is fairly common. It is when an entity becomes strongly self-willed and doesn’t care about wider harmony. Traditionally, the sin of Lucifer was Pride. To see one’s self as the equal of God.
Although quite a few occult groups see lucifer is personal deification, a light bringer, etc. Rudolph steiner saw it as one half of the inversion of God. Steiner saw the devil as being composed of Ahriman and Lucifer. Ahriman was a type of worldly ego that most people recognize; lucifer was a type of spiritualized ego, which most don’t really think about any more or even regard as a problem (such is the state of our culture).
An older view of things would usually regard most sins as mindless tendencies and addictions. Traditionally, a split between irascience/irascibility and concupiscence. But a worse sin, of an “angelic nature”, would be that of Pride. Steiner’s presentation of the devil would account for the various overexpressions of pride and self-will.
Swedenborg, a Swedish Christian visionary/mystic, had made a comment on sin. It is as though the sinner takes the power/force of (their share of) God’s will and turns it against the sweeping, constructive influence of God’s will. Either by being mindlessly emotive and, thus, destructive. Or being overly self-willed, at odds with the cosmic will. Both of these extremes were seen as a perversion of God’s will by an individual entity.
Thus, containment is not a correct term. God is the Creator. The Antecedent that is timeless and full of potential. But when different beings receive their share of the Infinite, it is their use that will create harmony (extending the dominion of heavenly influences) or create discord (extending enclaves for different demons).
**Edit, I incorrectly generalized/mixed ideas from different sources
