On the Mac, drag the link towards the notch at the top of your screen and let go on the person. Add a note first if you want one. The link lands in their Shareling library with your note.
You can drop on three kinds of target: a person (send), Self (keep it for yourself) or New person (start an invite). On iPhone and iPad, pick Shareling in the share sheet and choose who it goes to in the Send to row.
No. A Shareling channel is always two people, and there are no groups and no feed. One drop goes to one person. To send a link to three people, drop it three times; each copy can carry its own note. Your own library still shows the link once.
Yes, for one hour after sending. Unsend deletes the link from both libraries and leaves no "message deleted" marker.
After the hour, use Hide. Either person can hide any link at any time. Hiding removes the link from your own view only: nothing is deleted, and the other person's library stays the same.
You keep them. Before your channel with that person ends, Shareling copies every link they sent you into your own private space in iCloud and checks the copies. Each link keeps the sender's name, so your library looks the same afterwards.