Trying to understand page structures in PG

Hello

I understand that when an update of say 100,000 rows are made, PG
writes the updated rows as a new row. These new rows are not visible
to any sessions except the one creating it. At commit time PG flips
something internally to make these rows visible to all.

My Q: what happens to those rows which use to contain the values
before the update. Shouldn't something change in those rows to
indicate that those rows are no longer valid. Who does it chain those
rows to the new rows.

thanks.