Thank you for the question on the Unitarian understanding of John 1:1–14, “How have Unitarians understood verse 1 and verse 14 of the Gospel of John?”
The Unitarian view is there is only one God, playing three roles: God the Father, God the Son, and the God the Holy Spirit. It’s likened to an actor who changes his masks or appearance to be like another person, but it is the same person.
If I am not mistaken, the earliest Greek texts are written in capital letters without or nearly without any punctuation marks. The usage of “lower cases” was developed later. I cite a source for one’s consideration:
https://www.friktech.com/rel/canon/types.htm
The Early Church Fathers debated on the deity of Jesus Christ and his humanity in the Council of Nicea 325 CE. One view was condemned as heresy and the other view was uphold as being true. One can read about on the web.
Thus, to answer the question the Unitarians do not believe in the plurality of the Godhead, but one God being one person, playing three roles. John 1:1 is being interpreted that God played the role of become the Word who is later known as Jesus.
If the Apostle John wanted to say that the Logos was a created being of God, he could have used the word, “become” or “became” theos, God which would reflect creation. If John wanted to say that the Logs was being played by God, he could have used the Greek word, “hos” or “as.” John does not use those words.
Personally, I believe that the Logos is God while others may disagree with me. We are all called to study the text diligently.
-Kingston