iCloud & iCloud Photos · iCloud on the web, Windows and other apps
How do I move my files from Dropbox or Google Drive into iCloud Drive?
Short answer: Copy the files into a folder in iCloud Drive (making sure they're fully downloaded and you have iCloud space), let them upload, check them, then remove them from the old service. Storage Studio's Data Transfers can copy from a connected account into an iCloud Drive folder.
Copy the files into a folder in iCloud Drive and let iCloud upload them, then remove them from the old service once you've checked they're all there. Make sure the old service's files are fully downloaded first, and that you have enough iCloud storage for them.
With Storage Studio, you can use Data Transfers to copy from a connected cloud account into a folder you pick on your Mac — including one inside iCloud Drive — without opening each file. See copying from a cloud account.
Don't move the other service's own sync folder into iCloud Drive while its app is running; copy the contents instead — see using several clouds on one Mac.