How do Unitarians understand these verses which seem to say that all created things were created by Jesus? John 1:1-3: In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. Through him all things were made; without him nothing was made that has been made.
Colossians 1:16: For in him all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things have been created through him and for him.
Hebrews 1:8, 10: But about the Son he says, ... He also says,
“In the beginning, Lord, you laid the foundations of the earth,
and the heavens are the work of your hands. Revelation 3:14, &quot;The Amen, the faithful and true Witness, the Beginning of the creation of God.&quot;

These verses all say that it was Jesus or God the Son who created the universe. Isaiah 44:24 says:

Thus says the Lord, your Redeemer, and the one who formed you from the womb, I. the Lord, am the maker of all things, Stretching out the heavens BY MYSELF, And spreading out the earth ALL ALONE.

According to Unitarian theology Jesus Christ is a created being and He never preexisted His incarnation as a man. So explain how or why Jesus is identified as the Agent of creation according to the verses quoted?
John 1:1-3

The Word is further described in verses 9-18. There, note verse 18b: "the only-begotten god who is at the Father's side is the one who has explained Him".  Only-begotten, means had a beginning.

What about this one in Psalm 82:1, saying "God stands in the divine convention, pronounces judgment in the midst of gods."

Colossians 1:16

I had thought this explains the matter: all things created through [or "by means of"] him and for him.

Something similar is said of a personification of Wisdom in Proverbs 8:22-29, in existence before the physical creation as we know it.  Then it continues in v30,31: "Then I was beside him as a master worker. I was the one he was especially fond of day by day; I rejoiced before him all the time; I rejoiced over his habitable earth, and I was especially fond of the sons of men."

Would such a comparison be misplaced in a context where Wisdom represents Jesus?  Colossians 2:3 says "Carefully concealed in him [the Christ] are all the treasures of wisdom and of knowledge".  That notion brings the idea much closer to the ground, if I may say so.

On the master worker idea, if you build a house, you may do all the work yourself: design, obtaining materials, construction etc.  You may also choose to, say, let a contractor do all the actual construction works.  Although the contractor did all that work, who is said to have built the house?  Sometimes a bit of a grey area, isn't it?

Hebrews 1:8,10

These bits are much easier to understand if you take them in the context.  Just for a few highlights:

Verse 2 says that God had spoken "by means of a Son, whom he appointed heir of all things, and through whom he made the systems of things."

If Jesus was appointed heir, it means that he neither had been forever nor that he was naturally as God's equal.  After all, appointment is done by higher authority, not?

Verse 4 adds that "he [the Son] has become better than the angels to the extent that he has inherited a name more excellent than theirs".

The Son becoming better than the angels, implies that before that, he was at least in some respects equal to angels at the very best.  Is that something that could be said of the Almighty God, the Creator, who made the angels, that are part of creation?

Jesus as Agent of Creation

I cannot speak for every Unitarian, but Jesus himself did mention his pre-human existence in a number of occasions.  For example, John 6:38: "for I have come down from heaven to do, not my own will, but the will of him who sent me". Or in John 8:23, his own words "You are from the realms below; I am from the realms above. You are from this world, I am not from this world". The latter must have sounded weird and unreal for anyone who had his birth (or conception) in mind as the beginning of his existence.

But the most direct expression is in John 8, where the Jews ridicule Jesus for claiming he has seen Abraham while he's not even 50 years old, and his reply in verse 58: "Most truly I say to you, before Abraham came into existence, I have been".

Summary

So to sum it all up, Jesus may have been involved in the creation of all other things without being the Creator, just as God had spoken by means of him, in the same manner that God had spoken by means of the prophets.

I am not really familiar with Unitarian theology, but the Bible contains statements from Jesus about his existence before being human.

Context

There is a large number of passages and hints in the Bible that support the understanding that Jesus is not the Almighty God, for example the headship arrangement (1 Corinthiant 11:3), the prospect of the glorified Christ "subjecting himself to God" (1 Corinthians 15:28) and "the revelation by Jesus Christ which God gave him" [as opposed to Jesus having the knowledge naturally from the beginning] (Revelation 1:1).  Jesus also never supported that idea himself (Matthew 26:39; Mark 10:18; John 5:19-24; John 17:3; John 20:17,21; Acts 1:7).

Additionally, Unitarians question ideas like "What was the point of Jesus praying if he was speaking to himself?", or "if Jesus is God, how could he have died? How can God die? Or was he not really dead (as in not conscience of anything, Ecclesiastes 9:5.6,10), meaning that the resurrection isn't real?", or "How could Jesus have been sent as a messenger and granted things if he was of the same identity and had the same authority as the Father?", or "How could someone mediate between two parties, if this individual is a member of one of the two parties himself?" (1 Timothy 2:5,6; compare Daniel 2:44; 7:13,14).

With such a large amount of sources indicating otherwise, Unitarians do not lean towards changing their beliefs (into acknowledging Jesus as being God) just because a few verses are not entirely clear and may be difficult to explain. Besides, even unjust human judges are called gods in the Bible (Psalm 82:6). It does not seem irrational to me to say that a change in belief has to be backed up by sufficient evidence (more than a few verses that are open to different interpretations), and in harmony with the whole Bible.