1. The concept of phases in OVM/UVM arises because of giving symmetrical verification methodology, wherein all the phases have a particular usage. Now as we all know that classes are dynamic entities, we need to create the objects of them at run time. Build phase is for the creation purpose and is the first phase to be executed amongst all. In the build phase, you can also get the configuration(if any set in the top-level classes) and create your components accordingly.
2. uvm_sequence_item is a class extended from uvm_transaction and include methods in addition to methods provided by uvm_transaction. UVM has deprecated the use of uvm_transaction class and has asked to use sequence item class
3. you can see the usage of creating classes from create method instead of new while overriding the class. Else both methods will just create the object of classes.