The .7z extension indicates a compressed archive created using 7-Zip, a free and open-source file archiver.
The following article explains the two distinct components of this file: the (the container) and the Opus 55 musical works (the content). 1. Understanding the File Format (.7z) OP55.7z
7z uses the LZMA and LZMA2 algorithms , which generally offer much better compression ratios than the standard ZIP format—often 30–70% better . OP55.7z