Referring to the foundation, the New King James translation in Hebrews 6'1 refers to "the elementary principles" and shows the word "principles" in italics.
Translators use italicised wording for clarification where the Greek text seems to need it. Some don't and put words in as they see fit.
When the word principles is deleted what is left is "elementaries" or "elementary things".
That shows the description of the Believers' Kingdom Foundation, "the first principles of the oracles of God" and "the elementary things of Christ", as being twofold.
The point is, they are describing two different aspects, like understandings and experiences are aspects of a Believer's life.
The writer is saying that the principles of the oracles of God are about the elementary things of Christ.
But if principles are understandings, what are the Lord's elementary things that would, or should, be reflected in a Believer's foundation?
So what were, what are, the elementary things of Jesus for true Believers to understand and experience? A few scriptures reveal the things of Jesus, and Believers:
Jesus was born of the Holy Spirit - Luke 1'38. Believers are born again of the Spirit - John 3'3-8.
Jesus was immersed in water - Matthew 3'16. Believers are immersed in water - Acts 19'5.
Jesus had an experience with the Holy Spirit - Matthew 3'16. Believers receive the Spirit - Acts 10'47,19'2.
Laying on of hands and casting out demons are His elementaries, too.