This article discusses it at the beginning - https://atrainingtheologian.com/hebrews-hold-fast-essay/

'The author of Hebrews has been disputed throughout Church history. There are many suggestions yet it appears as though the author of this epistle is unknown.[8] Today the writer seems anonymous, yet at the time he was very well known by his readers stating his separation from them (Heb 13:19), how he wishes to see them (13:23) and calling his audience ‘beloved’ (6:9).'

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[8] 'In Harrington’s, What Are They Saying About the Letter to the Hebrews? He states, after reviewing much of the literature on Hebrews that is available, how nearly every scholar quotes Origen saying that only God knows the author of Hebrews. Unfortunately it is rarely told that Origen actually considered Paul to be the source of the material and that one of his disciples wrote it up for Paul. In the Letter to Africanus by Origen he explicitly states that he believes Paul is the source of this Frederick Crombie, trans. by, ‘CHURCH FATHERS: Letter to Africanus (Origen)’, in Letter to Africanus, July 10, 2016, http://www.newadvent.org/fathers/0414.htm, (accessed October 6, 2016), 10. Calvin points out ‘the writer himself confesses in the second chapter that he was one of the disciples of the apostles’ (Heb 2:3) Calvin, The Epistle, 1. Upon noting that the author is not Paul many have come up with various suggestions including Mary, Apollos and even Priscilla in Ruth Hoppin, Priscilla’s Letter: Finding the Author of the Epistle to the Hebrews (Fort Bragg, Calif.: Lost Coast Press, 1997). Luther’s suggestion of Apollos being the author does provide some plausibility. '

Ultimately we don’t know.

Questions about Hebrews authorship go back to at least Origen and Hippolytus. But by the late 4th century pretty much everyone agreed it was Pauline enough that the authorship was assigned to Paul and everyone dropped the question for a millennium.

Today, no one thinks that anymore. Ultimately although it has a lot of Pauline thoughts, it isn’t Paul’s vocabulary or grammar. The Greek really sounds nothing like him. And Hebrews is almost universally grouped in the non-authentic Pauline letters category.

The author gives a few clues to their identity. They’re an educated Greek speaker, and likely a native Greek speaker. They write after an educated fashion. And all Old Testament quotes are straight from the Greek Septuagint. They were from Italy or at least visited. They claim to know Timothy and say that they learned the gospel from those who first heard it. Finally they refer to themselves in the first person plural “we,” so there’s a good chance that it was a collaborative work.

So anonymous. Given the heavy Pauline influence and location, there’s a good chance we’re talking about one of Paul’s disciples. But which one? There’s no agreement. And there’s a decent chance that the author(s) is lost to history.

You may also want to ask this over at /r/AcademicBiblical and /r/AskBibleScholars . You’d get more answers and a more robust debate over at either of those subs.