Q: where are videos stored in 10.10.5?

Impossible to say without stating what videos you are referring to, are you referring to iTunes movies, iMovie, files you have downloaded from the Internet, etc? All are stored in different locations. Why not use Finder to search and then you can get a good idea. One thing I do is I tag every video I buy with a Tag called video. That way I can click that tag and get an instant idea where all mine are stored. If you are not aware of Tags then please read

OS X: Tags help you organize your files - Apple Support

Then they are stored in iTunes.

Are you syncing your iPhone to your Mac's iTunes by using a wired USB connection or by using iCloud?