I have designed a project using Arduino Every board which uses MCU Atmega4809 chip and I have used Arduino IDE for programming but now I am planning to design my custom board I have used schematics of Arduino every board available on Arduino website . how can i upload the bootloader (Where can I get the bootloader from ) and how can I upload the Arduino code on new Atmega4809 Chips.
Nano Every doesn't use a bootloader. The ATmega4809 on Nano Every is flashed with the SAMD11 used as UPDI programmer with Arduino firmware.
Optiboot supports ATmega4809 and MCUDude MegaCoreX supports different ATmega4809 configurations and Optiboot.