Here is an overview of my understanding. Questions are bolded.
The Protestant canon is 66 books.
The Catholic canon is 73 books.
Both agree on the 27 books in the New Testament.
The disagreement is what belongs in the Old Testament.
(I think:) There is agreement that the first 39 books belong in the Old Testament (although two of the 39 are lengthened by Catholics with additional content). The disagreement is about the books beyond the first 39 that were perhaps written later.
Catholics call these 7 books deuterocanonical. Protestants call these 7 books (and more - Martin Luther had 14) apocrypha. I'll call them the disputed books.
The Septuagint is an ancient Greek translation of the Old Testament (included some of the disputed books, although, for example, some codexes, according to Wikipedia, include Maccabees 1-4, 1 and 4 and no Maccabees whatsoever whereas the Catholics have precisely Maccabees 1 and 2). The Septuagint was written about 300 BC (although other websites I read made it sound like the disputed books were added to the Septuagint; also, as the answer below indicates at least some of them were written in Greek, vs the other 39 books were translated into Greek). The Septuagint is directly quoted in the Greek scriptures of the New Testament.
Something else, I think modern-day Judaism only holds the 39 in their canon.
One website claims the Protestant Old Testament canon is confirmed by the Councils of Jamnia in the first century: https://www.christianitytoday.com/history/issues/issue-43/how-we-got-our-bible-christian-history-timeline.html
90 and 118 Councils of Jamnia give final affirmation to the Old Testament canon (39 books)
However Wikipedia says this is disputed.
The theory of a council of Jamnia that finalized the canon, first proposed by Heinrich Graetz in 1871, was popular for much of the 20th century. However, it was increasingly questioned from the 1960s onward, and the theory has been largely discredited.
The council of Jamnia was after Christ. Allegedly it excluded Christians from synagogue. If the council of Jamnia is anti-Christian, clearly it wasn't led by Christians, so why accept anything the council says as authoritative?
Christianity Today (link earlier) says that the New Testament canon was established for the first time in 367.
367 Athanasius’s Festal Letter lists complete New Testament canon (27 books) for the first time
Before 367 was the canon of the New Testament disputed at all? Why did it take 300 years or so to decide the canon in the first place?
According to one website, the Catholic canon was decided in 382 AD:
The Catholic Church finally agreed on which writings should go into the Bible at the Council of Rome in 382 AD during the time of Pope Damasus.
(Edit: Wikipedia says Baruch was missing from the Council of Rome in 382)
Were there disputes about the canon before 382? If so, why did it take so long until the canon was officially decided at a council?
If the canon was decided in 382, does that mean there were no disputes about the canon from 382 until the Protestant Reformation?
Why did the Reformers dispute the canon in the first place if it was decided all the way back in 382? Were they saying the church had been wrong for 1,000 years straight?
Also, perhaps noteworthy, around this time the Latin Vulgate translation of the Bible was completed by Jerome (I think) and it contains the disputed books. Although something I read said Jerome believed these disputed books weren't Scripture. (Edit: some sources I've read say Jerome later changed his mind and believed the apocrypha, as he called them, to be scripture. He was translating the Old Testament from Hebrew Scriptures, which was a first, apparently, for latin Bibles, and the Hebrew Scriptures didn't have these additional books - which if they were written in Greek originally, that's not surprising that they were not in Hebrew)