Shimy the beliefs such as purgatory that are part of Catholic beliefs are in the “extra” books that you mention. Both the Catholic and Orthodox churches have more books in their Canon of scripture than Protestant. The reason is that at the time of early Christianity (literally the time of Jesus and his disciples and apostles) the common language of commerce, education and intellectual pursuits around the Mediterranean was Greek. Many Jewish texts were available in Greek and the early Christians wrote their new writings (letters etc.) in Greek for the most part. At this time the Jewish Canon of books was not fixed either, they also had extra writings which were used by various Jewish communities.
Therefore the early scriptures the first Christians used were Jewish and Christian texts mostly in Greek. At this time many Jewish communities could no longer speak and read Hebrew and they used a translation of the original Hebrew into Greek called the Septuagint (the name derives from the “70 translators” as there was a Jewish legend that 70 scribes had miraculously all produced a perfect translation into Greek).
The Septuagint was widely used by Jewish communities throughout the Mediterranean and it is this version that is quoted by Jesus and others in the New Testament.
Both Jewish and Christians in the early centuries did not have a final Canon (or a final decision of which books were declared inspired by God) of their Holy Writings. As well as the ancient Torah for example, various other writings were on scrolls and used by Jewish and Christian communities all around the Mediterranean.
The Christian Church at this time was united and gradually went through a process of discerning which writings and letters would be included in what became the Bible and the Old and New Testament. The books of what we know call The Bible were not clarified until 300+ years after the resurrection. The bishops of all the different churches throughout the Christian world met frequently in the 300s, especially in Carthage, North Africa. These meetings, called councils finally clarified the canon of what is now The New Testament. The Old Testament of the Bible was also clarified and some of the books which these Christians chose as part of their Old Testament were eventually a long time later rejected by the Jews and not included in their own Holy writings.
So basically the ancient churches such as the Catholic, Orthodox and Coptic churches still include in their holy scriptures writings which the early Christians in the first centuries decided were the inspired Word of God.
With the development of Protestantism in the 1500s onwards the leaders of these new movements chose to discard books that now were not included in the Jewish canon. So the Protestant churches removed a number of books that had been accepted by the historic church for over a thousand years. Within the books that were rejected by Protestants were scriptural basis for such doctrine as Purgatory and praying to the saints in heaven (although asking for those in heaven to pray for us Catholics believe is also in other parts of the New and Old Testament).
The word “Catholic” means universal and for 1,000 years although the Christian Church had to fight a number of heresies the church remained united as one body. There was a big split called The Great Schism in 1054 between the church of the West (Latin used in liturgy) and East (Greek used in liturgy). However the Catholics and Orthodox still share fundamentally the same beliefs. They both center their liturgy on two parts at every service
- Liturgy of the Word (scripture)
- Liturgy of the Eucharist.
So in summary the extra books you mention are still used in the ancient churches because Christians always accepted them as part of Holy Scripture (the Bible) until the Reformation. There will be many books discussing early Christian history. Basically there was always one church which sadly split in 1054 and then the other Christian denominations developed 500 years later.