The unitarian response from “The Trinity Delusion” (The Cliff Notes version 😉 )
1. In Hebrews 1:10, the quoted passage refers to God the Father.
a.) Because verse 10 starts a new kai/de argument.
b.) And we see that God the Father is the speaker in verse 13.
2. This makes sense, too, because the whole point of the argument is that Jesus is superior to the angels. And that He who made the heavens and the earth has not allowed angels to share His throne; but has allowed Jesus to have dominion over His creation.
a.) Compare 1:10 where it says, “And the heavens are the works of Thy hands” and 2:8 where it says “Thou has put all things in subjection under his feet.”
3. The other ways we know that verse 10 is not speaking of Jesus:
a.) It would break up the flow of the kai/de arguments (7/8-9, then 10-12/13).
b.) Trinitarians assume that, starting at 1:6 onward, the Father is speaking. (And so from verse 8 onward He is speaking of the son.) However, “He says” is translated from the Greek legei which usually refers to scripture so that it should read, “it says.” In verses 5 and 13 when God is actually speaking the verb eipon (eipen?) is used. AND God was not the speaker in any of the psalms from which these quotations are taken.