♥ or ⌥⌘C puts a copy in Favorites, where it stays until you remove it.
Favorites is a real folder, not a flag. Anything you put there stays until you take it out.
The copy in Favorites is independent: clearing your history does not touch it, and editing the original does not change it.
Open the Favorites tab, select the item and press ⌘⌫. The heart is one-way on purpose — you add from anywhere, you remove from inside the folder, so nothing disappears by accident.
A saved item keeps the first line of its text as a name. Open the preview, use the ⌄ menu and choose Rename to give it one of your own.
On the free tier the five newest favourites stay usable. Older ones are visible but greyed until you buy a licence, and nothing is deleted.
Did this answer it? If not, write to [email protected] and I will fix the page.