I'm not exactly sure what the question is, but I'll try to discuss it from what I understood.
You seem to be talking about opening emails (client software, or web client like gmail?) and saving attachments to the cloud. And you also mention not storing or storing it first on the pc. Well, if it's a client like Outlook, then most likely the attachment is already on your hard drive in some form or another. It might be encrypted inside the outlook pst file, or just straight up stored in some obsucre program folder. So whether you drag/drop it to another folder, it's already on your computer (but possibly inaccessible).
If it's a web client like gmail, the attachment is not on your computer. You can probably save it directly to the cloud if that feature exists ( i think it does in gmail). You might be able to save it directly to the cloud using a smartphone also, depending on the apps you are using.
I think the more common way with normal computer users is to place the file in your Dropbox or googe drive directory, which is an actual directory on your computer, and the software will sync it to the cloud. So in that case, yes you are first saving the file on your hard drive with drag/drop or whatever.
With zip files, you should also be aware that there are middlemen temp folders used during the extraction. So let's say you want to just open a document in the zip file through the email client...well, that has to first be extracted, and it most likely takes place in some temp directory on your hard drive. And it may also get deleted automatically when you're done with it.