Search engines like Google index the web by following links. When a server has directory listing enabled, the page is essentially a text file full of links. Google bots can read these links, index the filenames, and present them to users who search for specific terms. However, Google has increasingly de-indexed these malicious or illegal pages, making them harder to find.
By default, IDM limits its connection settings to 8 or 16 parallel streams to prevent overloading web hosts. However, power users regularly change this configuration to push the software to its absolute limit: parent directory index of idm 32
Open directories are often unmoderated. Always ensure you are downloading from a reputable source, and use an antivirus to check any .exe files before installation. Search engines like Google index the web by following links