How do I import photos from OneDrive or Google Photos to my photo library? (iOS)

I recently started paying for much more iCloud storage so I would like to import my photos from both OneDrive and Google Photos into iCloud so they’re accessible via my photos app. I can use their respective apps to download photos, but they aren’t imported to the Photos app. I have thousands of photos and videos, so it probably will be a tedious process. Also, I only have an iPad and iPhone, so Mac instructions will not be helpful.

It seems like I might have to download the photos to an external USB drive or card, then use an adapter to plug into my phone or iPad to do the import, is that correct? Isn’t there a way to just download them right into Photos? I don’t think that photos stored in the Files app appear in the Photos app.

Possibly the best option is to use a service like MultCloud to do a cloud-to-cloud migration. Has anyone done this?

Since I also pay for OneDrive storage, so it is storing the full versions of all my photos. Google Photos is more cleaned up, but it’s set to only store optimized versions, so I’m not sure that I want to use them.