Why is char[] on the stack but char * on the heap?

Arrays are not pointers. What your program is doing, line by line, is

// Allocate 6 bytes in the stack and store "hello" in them
char stack[] = "hello";

// Allocate pointer on the stack and point it to a static, read-only buffer
// containing "hello"
char *heap = "hello";

// Malloc 5 bytes (which isn't enough to hold "hello" due to the NUL byte)
char *heap_string_malloc = malloc(5);

// Reset heap_string_malloc to point to a static buffer; memory leak!
heap_string_malloc = "hello";

The reason you’re seeing the same pointer twice is because the compiler optimized away the second static buffer containing "hello".