When there is a change to postgres or redis. Knowing when this has happened is a matter of keeping track of things and is the reason that many people think that the two container setup is too hard. I have scripts that check postgres and redis versions and upgrade the data container if a change has been made. Those changed happen every year or two, though sub-base upgrades happen a bit more often. Generally if you don’t upgrade from 13.1 to 13.5 it won’t matter much.
Yes.