Scroll to the very bottom of an official video description and you will often find two hashtags sitting side by side. The first is usually just the act's name, something like #ActName. The second looks similar but carries an extra piece, like #ActName_SongName. We read the first as the general tag for the act, the one that covers everything about them. The second is a dedicated tag for that one release or campaign, so it points at a single moment instead of the whole catalog.
The underscore is doing real work here. A hashtag cannot contain spaces, so a tag written as the act's name, a space, and the title would break apart and only the first word would count. Joining the name and the title with an underscore keeps them together as one searchable word. When you type that tag into a search box, you land on posts that used the same exact string, which is the whole point of building it that way.
Official tags are usually listed only at the bottom of the description, which makes that the first place to look if you want to know which one the act uses. Searching the dedicated tag for a release keeps results about it together in one place, while the general tag mixes everything about the act into a single stream. Sometimes you will also see the same act's tag written twice, once in a local script and once in Latin letters. Those point to the same group, just spelled for different readers.
A few honest limits apply. A tag does not make a post official, because anyone can use any tag, so the uploader and the channel are what mark an official post. Tags can also change between releases, so copy the one from the newest post rather than an older one. Not every description lists tags at all, and the exact spelling and capitals may differ from one post to the next. Platforms generally treat capital letters the same, but they may treat an underscore as part of the word, so type it as written.