Where are my favourites saved?

If you are signed in: to your account, so the same list is there on any phone or computer you sign in on.

If you are not: in this browser only. They survive closing the tab, but a different phone — or clearing your browser data — starts empty. My Favourites says which of the two it is at the top of the page.

Sign in later and whatever this browser saved is added to your account rather than replacing what is already there.

A saved product also remembers what it cost when you saved it, so the page can tell you when the price has come down.