I don't understand a function of phrase and a relative pronoun

Solution 1:

Facebook has 3 billion users across its portfolio of apps, a massive number that has raised questions by some legal experts, including US lawmakers, about whether Facebook CEO Mark Zuckerberg neutralizes competitive threats by gobbling them up.

"A massive number" is part of a larger noun phrase that includes the relative clause "that has raised questions by some legal experts ..." functioning as modifier of "number".

"That" is a subordinator introducing the relative clause. The whole noun phrase "a massive number that has raised questions by some legal experts ..." is an ascriptive supplement that has "3 billion users" as its semantic anchor.