What does pipe `|` operator do in case expression of elm-lang?

The pipe is not a part of the case expression. It's record update syntax, as described here: https://elm-lang.org/docs/records#updating-records.

{ model | content = newContent }

assigns the value of newContent to the content field in the model record.

Read | as 'with'.

{ model 'with' content (set to) = newContent }