Arduino for STM32

hello. I am the my first time person dealing with 'STM32F103C8T6' this time.

There are 4 things I'm curious about.

Currently, I have purchased and used the 'Blue Pill' board.

1. I want to purchase the chip 'STM32F103C8T6' and make 'DIY, PCB' myself.
Q1) If so, how should I boot(bootloader)?

2. How to do a soft reset?
Q2) I am curious about the source code.

3. I want to test in conjunction with 'STM32F103C8T6' and 'DS3231'. By any chance, can I use the firmware as it is?
Q3) hardware I2C : PB_9-SAD, PB_8-SCL
Q4) Do you have any examples or methods?

Soft RESET

NVIC_SystemReset();
or
nvic_sys_reset();
or
HAL_NVIC_SystemReset();