How exactly SQLite database update primary key id?

Suppose I have a table contacts

  id        Name            Contact_No
-----------------------------------------
   1        abc                12345
   2        lmn                56784
   3        pqr                83654
   4        uvw                17637
   5        xyz                98345

If I delete row 3 from database, what would be the id of rows following the deleted row?

And if I update row 4, will it be given a new id or the id of row 4 will remain same after updating database?

What is the difference between autoincrement and autoincrement not null? Official documentation says that it will degrade the performance and database will work slow. Is it mandatory to write it along with integer primary key?

I have read many stackoverflow answers and also read SQLite Official Documentation but I couldn't get it.