There is 2 STM32 project :
ExtMem_Boot : The original project can be found in ST package This is more or less a bootloader, it boot from the internal flash, setup QSPI, clock, and external SDRAM, then it jump to external qspi flash.
Template Project : It's based on https://github.com/lvgl/lv_port_stm32f746_disco
The internal ram is allocated to Threadx pool (see _threadx_heap in the linker scrip) The frame buffer, (like everything actually) is place at the begining off the external sdram (see .LCD_Buffer in the linker script) This project build lvgl demo. The project create 2 task :
LiveLedThread_entry_function that blink the green led
Dma2d is disable for now, because lv_gpu_stm32_dma2d.c do not build.