Was it already well known what was and wasn’t scripture before the council’s.
So that the Catholic church just compiled what everyone already knew was scripture and needed no guidance from God in doing so?
The infant Apostolic Church possessed, copied and circulated the apostolic Letters as divinely inspired.
After the death of the last direct apostle to Jesus Christ (John), divine revelation ceased.
Now Sacred Tradition (the liturgical Mass) which encompassed the apostolic writing’s in the liturgy of the Word, practiced by the bishops who succeeded the apostles, when the apostolic Church suffers persecution and goes underground for the next 400 years.
Upon the persecution laws being lifted from the Catholic Church, She found that heretical and forged apostolic letters were being used within the diverse linguistic Liturgies.
The Church counseled and used a CANNON = (measuring standard), to prove which books were inspired of God and which ones were not. One of the measuring standards or cannon had to prove their books were used in their liturgies since apostolic times, authorized as authentic by an apostolic successor.
In short the Catholic church canonized the books of the bible for her apostolic Liturgy.
We have to take into consideration that
most people were illiterate for many centuries during and after the biblical age. The apostolic faith was handed down by Oral Sacred Tradition with the apostolic Letters being read to the faithful, just as it is done today in the Catholic Liturgical Mass unchanged.
To answer your OP; Yes those whom Jesus placed in authority over His Church and their apostolic successors as Bishops and Martyrs, knew the letters were divinely inspired and rejected those that tried to compete with the apostolic letters, but the populace probably did not, because they could not read. Yet, the latter received their apostolic faith by hearing and hearing the Word of God.
Peace be with you