Compiling error while compiling two java files in the same package that use each other

Suppose we have two .java files in the same directory.

Test.java:

public class Test {
    public void func() {
        System.out.println("Hello World");
    }
}

Main.java:

public class Main {
    public static void main(String args[]) {
        Test t = new Test();
        t.func();
    }
}

If I write in the command line: javac Main.java
it creates the Main.class and automatically creates Test.class and for running this program I use : java Main

but when I add package myPack statement in the first line of two files everything changes. and the structure of the files is as follows:

myPack
├── Main.java
└── Test.java

If I write javac Main.java the compiler says:

Main.java:5: error: cannot find symbol Test t = new Test();

and I should compile both of them at the same time. (javac Main.java Test.java)

Question 1) Why it happens?

Question 2) When I compiled both files I cant run program with java Main.

(Error: Could not find or load main class Main ).

What's the problem?

Question 3) I read somewhere you should compile and run this from the parent directory like this : javac myPack/Main.java (it automatically creates Test.class file) and then java myPack.Main . this way the program works properly. but why should we compile and run programs from the parent directory with this syntax?