how do I copy a video to my iPhone 6 Plus using iTunes ?

I have several movies I've taken with my iPhone 6 Plus of my Grand kids and transfer them to my iMac into "photos". But when I try to Sync them back to my iPhone using iTunes it will not let me do it.

So my question is: How do I transfer videos using iTunes from my iMac to my iPhone 6 Plus ???

My email is: S******@icloud.com

Please help.

Dave

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