There are two very separate issues.
First.. how the canon came to be, and how we can know it is inspired.
Second and completely separate -Can scripture stand alone?
Here the Catholic and early church view,
The problem for those who believe the second is they have no basis to decide the first. Although scripture gives the answer to both.
Jesus did not give us a book , he gave us apostles and successors to hand down the truth, a process known as " paradosis" sometimes translated as " tradition". They were not instructed to write and history shows most apostles did not.
They passed on the truth by word of mouth and letter,
Tradition carried not just the words but the meaning of doctrine. Take the Eucharist.. there is no doubt that the early church handed down from first generations the real presence in a sacramental Eucharist, believed to be the real flesh of Jesus. valid only if performed by a bishop, read ignatius and polycarp to see what was taught by John the apostle .
And there's the problem with sola scriptura - scripture needs valid meaning to interpret,You cannot do what reformationists have done and assign any meaning you think scripture can mean. They have many conflicting views for example on Eucharist and baptism because they lost tradition that carries the meaning of scripture, indeed they disagree on every aspect of doctrine,
The answer is authority,
Jesus also gave the apostles and successor of Peter the power to " bind and loose" which meant to the Jews the power to give definitive doctrinal interpretation.
THAT was the power exercised at council, the authority by which we know the canon is true. And what is valid scripture, Which is also why scripture says " the pillar of truth is the church" ( not scripture!) , hence scripture proves sola scriptura false, it points at truth outside of it.
In short - nowhere in scripture does it say it has to be in scripture to be true. So disproving sola scriptura by logic. So the authority of the church " the pillar of truth" , held by valid succession from early times is how we can know the canon is true, and also what doctrine is true,
At philosophical level..Scripture is " materially sufficient" not formally sufficient.
And is also why reformationists fracture into thousands of denominations and disagree profoundly on every aspect of doctrine, they have no authority to ask.so schism repeatedly on doctrinal difference.
How do you even determine what is a Bible? What books belong in a Bible? Is Tobin a legitimate book of the Bible? Is the Gospel of Mary? The Gospel of Peter?
How do you determine which translation to use? How about which translation not to use? Does King James' tinkering with certain verses make his version illegitimate? What about his amoral lifestyle or that of some of his translators?