The word translated “in its Maker” (בעושיו) shares a common root with עשיה, the lowest level of creation. With this abode in particular ought Israel rejoice, knowing that G‑d’s joy is especially great when the creations in Asiyah, the very lowest world, become an abode for Him.
וזה שכתוב: בעושיו, לשון רבים
For this reason the plural form —בעשיו — is used.
The literal meaning of the verse is: “Let Israel rejoice in its Makers.” Why the use of a plural expression in reference to G‑d
The Alter Rebbe explains that since G‑d is spoken of here as the “Maker” of the world of Asiyah, the domain of kelipot whose nature is arrogance and therefore separation and self-centeredness, the Divine creative power is referred to in the plural — for it is fragmented, so to speak. There is a multitude of created beings, each separate from the other, each animated by the Divine creative power; hence, a plurality of “Makers”, so to speak.
But this fault becomes a cause for still greater Divine joy, when these separate beings at the level of Asiyahunite in G‑d’s unity. This unification of creation is another achievement of man’s faith in G‑d’s unity, for this faith subdues the sitra achra which causes disunity.
As stated above, it is the earlier darkness which enhances the light that replaces it. Thus, the greater the darkness, the more superior the subsequent light.
In the Alter Rebbe’s words:
שהוא עולם הזה הגשמי, המלא קליפות וסטרא אחרא, שנקרא רשות הרבים וטורי דפרודא
This plural expression — “Makers” — refers to our physical world that is filled with kelipot and sitra achra, which are called “a public domain,” i.e., a domain of multiplicity,and “mountains of separation,” in that they are arrogant and separate from one another.
ואתהפכן לנהורא, ונעשים רשות היחיד ליחודו יתברך, באמונה זו
G‑d’s joy in the fusion of this plurality is aroused whenthrough this faith in G‑d’s unity they (the kelipot) are transformed into light, and they become a “private domain”— i.e., a unified realm — for G‑d’s unity.