Help needed to map the history of work item changes

When a work item is changed, from the front end, a history is maintained of all recent changes.
I can see that entries are made into REPOSITORY_CHANGE_EVENT
also I can see that changes are recorded in
REPOSITR_CHNG_VNT_STRNG_XTNSNS
but I can not find the link to how/where the "before" and "after" values of the changed item are stored?
This is prior to the snapshot action.

Can anyone help explain this data link.
Many thanks in advance.