To add to the good advice from @ ku4hx, if all of your vids live within albums in the Photos app, you'll copy the Photos Library file that is stored by default in the Pictures folder of your home folder.
Move your Photos Library...on your Mac - Apple Support:
https://support.apple.com/en-us/HT201517
If you have vids that are not within Photos albums and are scattered about your drive, you'll also have to copy/past those to a location on your external drive and carry that to your new iMac.
Whatever method you use, please do not delete the original files or wipe the original drive until you are certain and have confirmed that the images are accessible on your new iMac.
Now might also be a good time to make a fresh Time Machine backup of the old Mac and put it aside "just in case".
Back up your Mac with Time Machine - Apple Support