Hi Ziggityzaggity
welcome to the Apple (user-to-user) Community.
Re: How do you add an entire album of clips to an iMovie project on iPad to iPhone at once?
Wondering which version of iMovie you are using, and which iPhone iOS and iPad OS version is installed on the devices.
The info below is also available separately for iPhone also ... not easy to know how you are working between the two devices, as both can use iMovie.
With no experience in creating iPad OS or iPhone iOS movies, would like to offer what Apple has online:
You might want to try Apple Supports article (link below):
(Seems to say you can select multiple clips before you create an "magic" iMovie with them.)
Create a Magic Movie project in iMovie on iPad – Apple Support
says:
"Tip: To use all the media in an album you created, tap the album, then tap Create Magic Movie in the center of the screen. All the media in the album is added to your Magic Movie in chronological order."
Else:
With a movie that has already been created, seems on iPad OS or iOS you can only add individual clips, (yet you may be able to edit / trim each clip before you add the clip to the movie timeline instead of editing in iMovie after adding all the clips):
NB: For iMovie 3.0.1 ... else choose your version at the top of articles
Add clips and photos to Magic Movies in iMovie on iPad – Apple Support
Import media to iMovie on iPad – Apple Support
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Re: "iPad to iPhone":
Share to other devices in iMovie on iPad – Apple Support
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More info: iPad: iMovie User Guide for iPad – Apple Support
iPhone: iMovie User Guide for iPhone – Apple Support
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Also: Wondering if you are familiar with the (free) clips app ..
Seems you do have to tap each clip yet you can add them all at once
(Do not know if you can open an Album and choose "select all" on iPad)
Make a video in Clips – Apple Support
More info:
Clips on the App Store:
https://apps.apple.com/au/app/clips/id1212699939?platform=ipad
and
How to: Edit clips in Clips - Apple Support
Clips User Guide - Apple Support
All the best :-)