אַתָּה־הוּא יְהוָה לְבַדֶּךָ ...׃ - You alone are YHWH... Nehemiah.9.6
YHWH is God's proper name as God openly testifies:
... וּשְׁמִי יְהוָה לֹא נוֹדַעְתִּי לָהֶם - but I did not make Myself known to them by My name יהוה. Exodus.6.3
If YHWH is a proper name, and a very special one, it's obvious that's there's only one YHWH.
So what's the meaning of the verse then?
In Exodus 6:3, Moses learns that G-d appeared to the patriarchs “as El Shaddai, but I did not make myself known to them by my name y-h-v-h.” In the Bible G-d is called Y-h-v-h, “the Tetragrammaton,” a four-letter word. Many read this word and assume it is G-d’s name, but this is mistaken. The term Y-h-v-h, “the Tetragrammaton,” is a description of how G-d acts (see Exodus 3:14). So why did G-d reveals Himself as El Shaddai but not y-h-v-h? Nachmanides offers a possible solution.
According to Nachmanides, there exist not only open miracles, like the parting of the Sea of Reeds, which is evident, but also hidden miracles that occur daily. For example, falling leaves, rain, and snow, because G-d is the source for every fall of each leaf, rain drop, or snowflake. He writes that this is the “greatest secret” of the Torah. Nachmanides understood G-d to be saying that He supplied the patriarchs with hidden miracles but He will now save Israel from Egypt with open miracles that disrupt the current establishment of the world so that everyone will know that G-d saved the Israelites from slavery.
See https://www.sefaria.org/Bava_Batra.25a?lang=bi
אלא מאי תדירא תדירא בשכינה דאריב"ל בואו ונחזיק טובה לאבותינו שהודיעו מקום תפלה דכתיב (נחמיה ט, ו) וצבא השמים לך משתחוים
Rather, what is the meaning of frequent? It means frequent with the Divine Presence, i.e., the Divine Presence is found on the western side, and therefore it is inappropriate to set up a tannery there with its foul odors. As Rabbi Yehoshua ben Levi says: Come and let us be grateful to our ancestors who revealed to us the place of prayer, as it is written: “And the hosts of heaven bow down to You” (Nehemiah 9:6). Since the celestial bodies move from east to west, they bow in that direction, which indicates that the Divine Presence is in the west.
ורבי אושעיא סבר שכינה בכל מקום דאמר רבי אושעיא מאי דכתיב (נחמיה ט, ו) אתה הוא ה' לבדך אתה עשית את השמים וגו' שלוחיך לא כשלוחי בשר ודם שלוחי בשר ודם ממקום שמשתלחים לשם מחזירים שליחותן אבל שלוחיך למקום שמשתלחין משם מחזירין שליחותן שנאמר (איוב לח, לה) התשלח ברקים וילכו ויאמרו לך הננו יבואו ויאמרו לא נאמר אלא וילכו ויאמרו מלמד שהשכינה בכל מקום
The Gemara comments: And Rabbi Oshaya holds that the Divine Presence is found in every place, as Rabbi Oshaya says: What is the meaning of that which is written: “You are the Lord, even You alone, You have made heaven…You preserve them all alive and the hosts of heaven bow down to You” (Nehemiah 9:6)? This indicates that Your messengers are not like the messengers of flesh and blood. The messengers of flesh and blood return to the place from where they were sent to report on their mission. But Your messengers return and report on their mission from the very same place to which they are sent, as it is stated “Can you send forth lightnings, that they may go out and say to you: Here we are?” (Job 38:35). The verse does not state: They will come and say, i.e., they do not return to their point of departure, but: “They may go out and say,” which teaches that the Divine Presence is found in every place.
According to the gemara, the passuk comes to define the "place" of the shechinah.
Therefore the reference to Hashem in the beginning of the passuk seems to be a general statement that is refined by the following references.
אַתָּה־ה֣וּא ה֘ לְבַדֶּךָ֒, אַתָּ֣ה עָשִׂ֡יתָ אֶֽת־הַשָּׁמַיִם֩ שְׁמֵ֨י הַשָּׁמַ֜יִם וְכָל־צְבָאָ֗ם הָאָ֜רֶץ וְכָל־אֲשֶׁ֤ר עָלֶ֙יהָ֙ הַיַּמִּים֙ וְכָל־אֲשֶׁ֣ר בָּהֶ֔ם וְאַתָּ֖ה מְחַיֶּ֣ה אֶת־כֻּלָּ֑ם וּצְבָ֥א הַשָּׁמַ֖יִם לְךָ֥ מִשְׁתַּחֲוִֽים
Looking at the verse in its entirety I believe there's a figurative element to the language, attributing the [act of] creation to G-d alone, rather than the designation’YHWH’, as is suggested by the English translation.
I would offer a slightly less literal translation of the verse:
Thou alone, YHWH, [Art Who] hast made the heavens, the highest heavens, and all its hosts, etc.
