1.Does undo buffer exists or changes will directly write to undo datafiles?

Undo blocks are database blocks, so they will sit in the buffer cache like others, eg

SQL> select count(*)
  2  from v$bh
  3  where file# in (
  4    select file#
  5    from   v$datafile
  6    where  name like '%UNDO%' );

  COUNT(*)
----------
      5473

2.Does redo contains both undo & redo related changes?

Yes. Any change to any block (undo or otherwise) is protected by redo (unless explicitly instructed not to)


3.If db crashes with uncommitted data in undo buffer it will get cleared automatically ,then we are able rollback database with redo ?

During instance restart, we do instance recovery. We use the redo logs to resurrect all the changes to database blocks, which *includes* undo blocks. Those undo blocks can then be used to undo any uncommitted transactions (ie, uncommitted at the time of the crash)