From what I understand, it depends on the stage. The United States Code certainly tracks any and all amendments, and you can fully trace which member introduced which amendment, when and where it passed, and even verbatim floor debate.

However, the draft stage isn't documented this way. Members negotiate whatever between themselves (well, really their staffers) and this happens over email, in discussions, via Word documents - whatever works.

I guess in the git metaphor, drafts are in flux while being worked on as a commit, and are squashed and then accessible as such squashed commits once initially introduced or whenever they lead to bill amendments. You can't necessarily track down what member was responsible for a specific sentence in an amendment.

The thing is that there is a voting process eventually so it can at least in principle be known which version is proposed by which legislator, or which committee.