Spoilers on Lemmy work like this:

Text next to the arrow
Text inside of the spoiler

which can span multiple lines and can contain *formatting*

which produces this:

The first word after the three colons must be "spoiler" otherwise it won't work.

Support for these in mobile apps is somewhat lackluster. Sync for Lemmy did not really change their formatting code at all since they stopped being a Reddit client and virtually none of the Lemmy specific formatting features work. Voyager does not render spoilers at all. They work just fine in Jerboa though, as well as on the web (desktop and mobile browsers).