How to understand BeginPackage and Begin properly?

Question

Using Get to evaluate these three .wl:

First:

BeginPackage["Functions`"]
S1=1;
Begin["`Test1`"]
K1=2;
End[]
EndPackage[]

Second:

BeginPackage["Functions`"]
S1=1;
Begin["Test1`"]
K1=2;
End[]
EndPackage[]

Third:

BeginPackage["Functions`"]    
S1=1;    
Begin["`Test1`"]    
S1=2;    
End[]    
EndPackage[]

In first code, using ? can get Functions`S1=1 and Funstions`Test1`K1=2 properly.

But in second code, only Functions`S1=1 could be found, where is Funstions`Test1`K1=2 or Test1`K1=2?

In third code, why the result is Functions`S1=2 not Functions`S1=1? And where is Funstions`Test1`S1?

How can I understand these differences.

Sry I'm newbie and not good at English, the question may not be described appropriately.