alwayson read only routing if i'm change connection string to applicationintent:readonly then i setup read only routing list in my SQL server 2016 , does that mean my application will route all write request to primary and then read only request to secondary server which i setup as read intent only?
No, it means connections with applicationintent:readonly will be routed to the readable replica. If one of them tries to modify data it will get an error message. Something like below:


  Msg 3906, Level 16, State 2, Line 2
  
  Failed to update database "XXX" because the database is
  read-only.


Note: if the secondary readable replica is not available the connection will be able to be established to the primary one depending on Connections in primary Role option.