Title says all. If I use online youtube downloader, is there any risk of putting my computer into danger? Like the video / audio file can contain dangerous executable code?
Potentially. Or they could be fishing for personal information, or any number of other nefarious intents.
Or they could just be out to make ad rev off you with popups, etc...
But there's no need to risk such potential threats/annoyances - just use youtube-dl (https://github.com/ytdl-org) - there's plenty of tutorials out there on how to use it.