I searched for articles on the internet for multi-camera editing, but I can't find "Create a new multi-camera clip with the selected clip". Please tell me why and how to use it.
Perhaps the article you referenced is from version 16 or earlier when DaVinci Resolve has a cut page.
In addition to the "edit page" that existed before, version 16 has added a "cut page" specialized for cut editing.
"Create new multi-camera clip with selected clip" can only be selected in "Edit Purge".
If you are editing in the "cut page", switch to the "edit page", select multiple clips in the media pool and right click to bring them up.