When you choose Share, one of the options in the sheet should be Save File (or similar). Once you choose this you should get a file browser that lets you navigate the file system (such as it is). If you do not see iCloud as an option the tap the left top button, if there is one (and it does not say 'Cancel'). Eventually this you'll see iCloud as an option. Select this and navigate to the folder you want to save the file in.
There are other methods depending on what app you are using. Not all will provide the Save File option if they have other options. Almost all of them will provide a simlilar file browser if they allow saving the file outside of the app at all. If you use the Files app, you should be able to just drag the file into iCloud, or select it then choose Move, or Copy if it's available.
Unfortunately there is no universal place to find Save File or other save options, or even completely standard file browser. Even Apple's apps are not 100% consistent. You may have to poke around a bit.
Keep in mind that trying to move a file from one service to another only copies the file. The original will not be deleted. Move only works as expceted when moving a file from one folder to another on the same service/device/storage device.
There is no explicit uploading to iCloud. You save stuff to iCloud, and it syncs to other devices that also have iCloud enabled (assuming they are all on the same account). So, once you copy something to iCloud on the iPad, just open the iCloud folder on the Mac. There may be a delay, but shortly the file should appear. If the file is really large, you may have to wait a bit.
You can also access your iCloud Drive using a web browser and going to the icloud.com site. If the files are not showing up on the Mac, this is a good way to see if the iPad is copying to iCloud, so you can trouble shoot wether it's the iPad or Mac that is not syncing correctly.
I'm sorry if I remember incorrectly, but if it was you who was trying to transfer large number of files using AirDrop, using iCloud may still be a less than satisfactory way to transfer large numbers of files to the Mac. Well, unless you want those files to be on both devices and to stay synced, Then it is the best method. But it probably means you'll need more than the 5GB if free icloud storage Apple provides.
There is a way in the Files app to access folders shared on a PC, similar to how you share folder between Windows PC's.
Go to the PC and find out what its IP address is. In the Files app on the iPad tap on the three dot (...) icon and choose Connect to Server. Enter the IP address, then enter a valid account on the PC. Any folders that have been shared on the PC (for that account) should show up under a new Shared header in the left column. If there are no shared folders you'll need to set them up on the PC. I'm no expert there, so you may have to do some research.
A bit of a pain, but once you set it up the iPad will remember the address so you don't have to enter it again., You'll probably have to sign back into the account occasionally. Probably every time the iPad leaves your network.
