How many days? when does covid virus reaches the lungs? how long it takes to reach there from the onset of symptoms?

Likely day one. Covid is a respiratory virus. Much of the evidence shows airborne transmission more common than from touching contaminated surfaces and transferring it to your face/nose/mouth. When it is inhaled, it may start in your lungs or move there within hours. The timing of symptoms, if they even occur, depends on individual response factors. It could be hours/days or never.

It has been my understanding that the virus remains in the throat during the incubation period for a variable period of days. Our immune system nay contain it until the virus is able to replicate and then may or may not proceed into the lower respiratory tract in some, but not all, people. Some fortunate people prevent this replication. A virologist can your answer best, however