To Turn Cloud Syncing on in Lightroom Classic CC....
Click in the upper lefthand corner of the Lightroom application where your name appears. Of course you should have already signed into your Adobe Account.
Then click on the "Sync with Lightroom CC".
It looks like this when you have it turned on with PAUSE icon to stop syncing.
You can only sync to one Catalog. This usually isn't a problem. I have mine synced to my desktop computer and not to my laptop. If I have to change that for some reason, all the images will be removed from the cloud and the new choices will have to sync up.
To make your collection visible to others online...
Right Click on the collection and choose Lightroom CC Links. Then you can copy the URL link and share it.
To sync a Collection back to the CC version, click the the checkbox to the left and a lightning bolt icon will appear. The Jackson 2017-1209 Collection is synced. Note that my Jackson collection has a little note icon. That means it garnered comments from viewers. If the viewer has an Adobe ID, they can add comments to images.
Images from your Lightroom CC on your phone can sync to your Classic application as well. Note that the location for those files can be chosen. I have them going to my "Working" folder then organized by date.