If the Allah is One, why does Allah say "We" in the Quran?!

Allah is one, yet, some verses of the Quran refer to God in the plural form? Why does Allah say "We" in the Quran? For example:
We built the heavens with Our power and will continue to expand them (51:47)

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======== AMAZING QURAN STORIES ========
Do you enjoy stories? How about stories that teach you about the Quran?
If you answered yes, then watch: "Amazing Quran Stories You have Not Heard Before! Best English Tafsir" https://www.youtube.com/watch?v=beOvtLWVRwM

======== RECOMMENDED PLAYLISTS ========
• Al Imran Tafsir (New format) https://www.youtube.com/playlist?list=PLS0yR427QfJ__R8Bka7a03X4pC1hrWjnt
• Quran Tafsir Journey (Old format) https://www.youtube.com/playlist?list=PLS0yR427QfJ8yD8CyETOdzV_Ohyz_trpA
• For New Muslims https://www.youtube.com/playlist?list=PLS0yR427QfJ-_2b5XFmOLCPLG9P6V-dtr

======== QURANGARDEN LINKS ========
• Website: http://www.QuranGarden.com
• Podcast: https://podcasts.apple.com/us/podcast/quran-garden-holy-quran-explained-in-clear-english/id1209725805
• Google Play/Android App: https://play.google.com/store/apps/details?id=com.qurangarden.qurangardenapp&hl=en_US&gl=US
• Apple Stor/iOS App: https://apps.apple.com/us/app/quran-garden-english-tafseer/id808878972

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