aum undo tablespace quesiton

 aum  undo tablespace  quesiton :

we kown in aum  undo tablespace  will auto create ,delete , online ,offline undo segment,

the undo extent in undo segment will have expired , unexpired,atcive .

i have several question to ask :

1.  if we kown a  undo extent  is  unexpired, how dose we kown what data store in it , how long  time  will it  expired ?

2.our customer said they find undo tablespace usage is 90%, they restart db , and still is 90%, so can we have a method to decrease their usage?

3.does one undo segment in one time can only allocate to one transcation use , or one undo segment  to more undo segment  at a time  ?