A in both pieces of code is initialized by default. The difference is that the global variable a will first be zero-initialized (zero-initialized) before the default initialization, while the local variable a will not (this does not mean that the local variable will not be initialized to 0). At the same time, the value of this default-initialized local variable a is uncertain. So the value of global variable a here is 0, and the value of local variable a is uncertain.

Whether a variable will actually be initialized to 0 is related to the storage duration of the variable. Static variables are initialized to 0 before initialization, while automatic variables are not.

8.5 Initializers
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5 To zero-initialize an object or reference of type T means:
— if T is a scalar type (3.9), the object is set to the value 0 (zero), taken as an integral constant expression, converted to T;
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— if T is an array type, each element is zero-initialized;
6 To default-initialize an object of type T means:
— if T is a (possibly cv-qualified) class type (Clause 9), the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor );
— if T is an array type, each element is default-initialized;
— otherwise, no initialization is performed.
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9 [ Note: Every object of static storage duration is zero-initialized at program startup before any other initialization takes place. In some cases, additional initialization is done later. —end note ]
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11 If no initializer is specified for an object, the object is default-initialized; if no initialization is performed, an object with automatic or dynamic storage duration has indeterminate value. [ Note: Objects with static or thread storage duration are zero-initialized, see 3.6.2. — end note ]

Because undefined global variables will be initialized to 0, but local variables declared within functions do not have this rule

Generally speaking, global variables and static variables are placed in the data segment, while local variables are placed in the stack.
The data segment is determined during compilation and allocated in the executable file, so it will definitely be initialized.
As for the stack, we know that it is allocated at runtime. When entering a function, in addition to registers being pushed onto the stack, stack space is also allocated to temporary variables, so local variables correspond to a stack address. The stack changes greatly in memory, so the initial value is uncertain.
Of course, the C++ standard does not specify which area the compiler should place variables in, but this is generally how it is implemented.