How do Torah only people determine what is The Torah?

People like the Remember the Commands YouTube Channel keep saying things like "The Torah never told us to recognize the Canon of Jamnia, or the Calvanists".  My response is....

How do you know the Torah is what we today think it is?  Nothing in The Torah defines it as Five Books of Moses, one of which is actually a historical narrative of events that happened before Moses was born, so traditionally saying Moses wrote it kinda hurts seeing it as an eye witness account of that history.  And another is a book Moses couldn't have physically wrote himself since it records his death.

Now Deuteronomy can be called a "Book of Moses" since it's mostly a record of Speeches Moses gave, so quoting it for the most part is quoting Moses.... IF it's an authentic account.

Only Deuteronomy refers to there being a "Book of Law" in that exact phrase.   You can interpret those verses as implying what your reading now is part of that book, but that's kinda iffy.  It seems equally likely it's referring to a book already written.

Why would a book of Laws also include all these historical narratives?  Well there being commands to remember things seemingly makes the historical narratives part of the Law.  But still, one could easily argue that The Torah proper is just what God gave to Moses on Mount Sinai.

Exodus 20:22 begins the proper Covenant Law Code, which arguably concludes in Exodus 24:4, this was written in a Scroll by Moses.  But Exodus 25:1 through 31:17 are seemingly what is defined as being written in Stone, according to 31:18.  The Stones had to be carved again after the Golden Calf incident.  But Exodus 34:28 says the second set of Tablets were written by Moses dictated by Yahuah not by the Finger of Yahuah like the first Tablets were. 

Leviticus is stuff added later, as are the parts of Numbers that are new commands, they don't even define themselves as part of either the original Scroll of Moses or what was written in Stone.  Deuteronomy meanwhile, doesn't even pass it's own Prophet Test by the standards of how Remember the Commands interprets things.  But even if presumed to be a valid record of Speeches Moses gave at the end of the wandering, it mostly demonstrates that Moses memory was often faulty, even the Ten Commandments are expressed differently.

I'm certain Deuteronomy is Canon, since I think anything Jesus quoted as Scripture must be Scripture, and Jesus quoted Deuteronomy 6 for what He called the Greatest Commandment.  In fact Deuteronomy is the most quoted book of The Torah in the New Testament, with even some NT references to the Ten Commandments seemingly preferring Deuteronomy's version.  Jesus also quotes Genesis, Exodus and Leviticus, and allusions are made to events from Numbers.

Thing is, I think it's fairly illogical to accept Deuteronomy as Canon but not Joshua.  I think Deuteronomy and Joshua's scrolls mostly had the same human author, much of Deuteronomy seems like it's written there just to set the stage for Joshua.