Missing remote media and iCloud confusion in iMovie on iMac

I just got an iMac because I was told it would be easier to make home movies of my photos and videos of my grandkids on this vs buying another windows computer since I have an iPhone. Well let me say it has not been easy. I have been trying for days now to create an iMovie and sometimes it seems to go well but then all of a sudden videos are not showing or won't play and it says ?missing remote media. I was told that iCloud and my photos were one and the same? I really hate iCloud and don't know if I have to download them still from the "Cloud" or what. Can anyone help?

[Re-Titled by Moderator]