Most discovered manuscripts are in fragments. Only a few manuscripts are complete. Scholars take pieces to create the complete picture or puzzle from the 2nd Temple period: end of the exile in 530s BCE till the destruction of Herod’s Temple in 70 AD.
The Great Isaiah Scroll in cave #1 was 99% intact. This Isaiah scroll is the oldest extant copy of the book of Isaiah. However, even this Great Isaiah Scroll itself was copied 400-500 years after Isaiah. So, even this copy was a copy of copies of copies.
1/3 of the text are from the Old Testament. 2/3 of the Dead Sea Scrolls are legal or other extra-biblical texts. We can read these texts like we read the Apocrypha or Pseudepigrapha.
Most of the scrolls were found in cave #4.
A great place to start:
Lots of wonderful perspectives: