VirusTotal runs a file, a URL, or a hash through many engines and leaves the result readable. It is not an antivirus on your disk. It is a look at whether this exact file has been seen, and what the scanners said.
Check it before you run a download. Start with the SHA-256 hash. If a report exists, you see detections, the first-seen date, and the names the file has used. A known installer build looks different from a file seen for the first time today.
A green bar is not a verdict. Engines miss in both directions: fresh malware can score zero, and your own packed tool can score red. Read the ratio and the context, not a single icon.
Extension
The official extension is VT4Browsers (Chrome, Firefox, Edge). From the context menu you can check a link, and a download can go to a scan without a separate visit to the site. On a page of logs the extension highlights hashes, domains, and IP addresses. Description and stores: Browser extensions.
The defaults send some downloads. Open the options and decide what is allowed to leave the machine.
Desktop app
A windowed uploader does the same job without the browser: a context menu on Windows, drag-and-drop on macOS. List: Desktop apps.
What not to upload
An upload is public. The file reaches the engines and stays in the corpus. Query the hash first. Do not upload client disks, keys, samples from a real case, or anything you would not place on another company’s server. A hash lookup, without sending the file, does not publish that copy.