I want to implement FOTA for stm32u5 using bootloader, I am new to the ST ecosystem and it would be helpful if I get some light on this ??????

[MM..1]

You need specify FOTA over connected PC or FOTA on networked STM directly.

For first read AN2606 about system bootloaders and use STMProgrammer utility

for second you need code ...

[Tesla DeLorean]

Outside of the STM32 domain, have you ever written a loader, or coded any type of firmware update?​