Server Administration :: Query On Redo And Undo Basic Concept

understanding a redo/undo concept . Refer following data

create table t(n number);
insert into t values(10);
commit;

now I update as following

update t set n=20;

As per my understanding the before image i.e. n=10 is stored in undo (to be used for rollback, transaction recovery and even in instance recover but not in media recovery) and after image n=20 is stored in redo (to be used for various recovery purposes including media recovery in case of consistent backup).

So it is redo logs for rolling forward and undo for rolling back making transaction, db consistent . If my above understanding is true then what is meant by the term 'redo required for undo'?

Also, if there are 2 database db1 and db2 connected using database link where we are populating t1 table in db1 using t2 table in db2 using db link where redo and undo will be updated db1 or db2?