In the Sukkah, we are all one because in essence we are a single being.
But that which is outside our essence—that which makes us the individuals who we are—that is not yet one.
In the unity of the Etrog, Lulav, Hadasim and Aravot, we are all one as distinct individuals, complementing one another to make a single, harmonious whole.
But we remain apart at the core, united only superficially.
When the two mitzvot come together, then we are one in all ways: The oneness of our essence resonates throughout the harmony of our diversity.
And so it shall be in the entire creation, as a single symphony, soon, very soon.
As human beings, we are each one with our own personality. Yet as Jews , we are one because the Torah unites us, into a specific mindset that starts with ONE creator H'. It took 40 years in the desert to truly unite the Jewish people so that the should feel as one - despite their individuality. There must be a balance between being unique as we are, and feeling part of a larger community qua Jews.
Hard to fathom how we are one!?
Yet we are , if we take Judaism seriously , and strive to the best of our ability to be part of that larger group: a few examples: when we give Tzedakah gathering for prayer, celebrating Yom Ha'atzmaut, planting trees in Israel.
H' is ONE, and the Jewish people ( as "Am Segulah" ) should strive to be one.