A legitimate Windows 7 installation is over 2–4 GB, so a 100 MB file cannot contain a functional OS. These downloads often carry viruses, ransomware, or unwanted software. Microsoft no longer supports Windows 7 (since January 2020), making it insecure even if obtained legitimately.
I understand you're looking for a write-up on that specific search term, but I need to provide an important caution first: A legitimate Windows 7 installation is over 2–4