Hello lootje98,
Thanks for posting in the Apple Support Communities.
It sounds like you're looking to download iCloud videos on your Mac to use with other apps, right?
If we have that correct, let's first check how much available storage you have on your iMac. If the available storage is low, this could be what's preventing the videos from downloading. Check out the steps below for assistance with reviewing the used and available storage, and you can find more details here: See used and available storage space on your Mac - Apple Support

Choose Apple menu
> About This Mac, then click Storage.
The categories shown contain the various files on your Mac. (If your Mac has more than one user account, the categories shown are for the user who’s logged in.)
Note: The categories do not correspond to specific folders on your Mac.
If you have enough available storage on your iMac, let's review the next steps for assistance with downloading videos from iCloud. You can also locate steps for other devices in the related resource, here: Download iCloud photos and videos - Apple Support
On your Mac or PC
1. On iCloud.com, click Photos and select a photo or video. Hold the command key on your Mac or control key on your PC to select multiple photos or videos.
2. Click and hold the download button
in the upper corner of the window. If you want to download your photo or video as it was originally captured or imported, choose Unmodified Original. For JPEG or H.264 format — including edits, and even if it was originally in HEIF or HEVC format — choose Most Compatible.*
3. Click Download.
If your photos download as a ZIP archive, learn how to uncompress the folder on your Mac or your PC.
We hope that's able to help. If not, provide more details on what occurs when you try to download the video. In addition, let us know if you're seeing any messages appear along with other details that could be helpful. We'll use this information to gain a better understanding of what's occurring to provide you with the best next steps to take.
Have a great day!