how to add music to itunes from folder

Probably File > Add Folder to Library. You can also drag & drop, or copy the media into <Media Folder>/Automatically Add to iTunes. It depends somewhat on the current location of the media files and your preferences for Keep... and Copy... under iTunes > Preferences > Advanced. I.e. do you want iTunes to make new copies of the content you are adding, or link to them where they are now? Do you want iTunes to be able to rearrange the files according to their properties? I would normally recommend you enable both Keep... and Copy... so that all of your media is organized within the iTunes Media folder.

tt2

Ah, I was saying what my general recommendations are in advance of knowing any particulars that might alter that advice. You also implied that this media wasn't already connected to the library, or at least didn't make it clear that it was. See Make a split library portable - Apple Community for general advice on keeping your media on an external drive. When using the Copy... option you should only ever add any given item of media that is located outside of the media folder to the library once. For content inside the media folder, or that is referenced on an original path outside of the media folder, iTunes is able to detect items that are already part of the library and not make unwanted duplicates.

See Duplicate songs in iTunes - Apple Community to clean up duplicates in future.

tt2