Is it possible to use Serial SRAM (QSPI) with the STM32H743?

Yes, but ...

• Only one of them can be accessed at the same time (but the RAM can be accessed while the flash is busy erasing/writing sectors)
• Memory mapped mode is read-only.
• It can be slow.

And if i use only SRAM

Is it possible to get the Access to QSPI SRAM like this?

Scatter File:

...
 RW_RAM2 0x90000000 0x00080000 {
 *.o (ExRAM) 
 }
...

I would like to access the section ExRAM like this:

...
uint16_t Array[1024] __attribute__((section("ExRAM")));
...

Thanks

As I have said, memory mapped mode is read-only, you can't directly write values into the array, but you can read them like that as long as you set up memory-mapped mode again after each write access.