Downloading the ISO file itself is not illegal. Sharing copyrighted software via torrents is copyright infringement. However, Microsoft has largely abandoned Windows 7.
The Internet Archive hosts thousands of abandoned software ISOs. Search for "Windows 7 Home Premium OEM ISO (Digital River)" – Digital River was Microsoft’s official distribution partner in the 2010s. These are untouched, original images. Crucially : Verify the SHA-1 hash before installation. windows 7 home premium oem iso