Some comments...
1. The "Book" Christians know is authoritative began long before Christ came to earth. 2/3 was written before that point.
2. The Apostles were alive during the completion of the completion of the NT Scriptures. It was widely known which books were inspired.
3. The Apostles carried authority as having been personally selected by Christ.
4. Solo Scriptura does not mean Christians do not recognize other authorities in the Church - the NT itself tells us to obey our church leaders, for example - ,but that all teaching must not contradict inspired Scripture, nor add to it.
Best.
But as I pointed out already, these same people take their day of rest on Sunday, which is a practice put in place by the Catholic Church; there is no authority found in Scripture for this practice.
We read as well in the New Testament how the Church, exercising its teaching authority in the form of the Magisterium (one of the other two pillars of authority) declares that circumcision is no longer necessary and that certain meats were no longer considered unclean. (This teaching authority is demonstrated to you in the Scripture you rely on and you won't concede it.)