when the session expires / browser crash happens the data will be stored in the operator's shared memory. When the requestor available the same data will be shown on the screen. these will be taken care OOTB,
I have copied the smaller content from the guide regarding this
"For initiated outages, such as a planned shutdown for maintenance purposes, the Pega 7 Platform persists requestors. This involves taking the entire requestor state and saving it to external storage that is shared between Pega 7 Platform servers, such as on a file system or database. This can be done using the default passivation mechanism or a custom passivation mechanism."