MCUs GPIO modules have at least 2 registers :

• Direction Register (called: Mode register by st)

  Set pin as input or output

• Data Register

  Write to pin or read from pin

#include "stm32f4xx.h"

#define GPIOAEN    (1U<<0)
#define GPIOCEN    (1U<<2)

#define LED_PIN    (1U<<5)
#define BTN_PIN    (1U<<13)

int main(void)
{
    /*Enable clock access to GPIOA and GPIOC*/
    RCC->AHB1ENR |= GPIOAEN;
    RCC->AHB1ENR |= GPIOCEN;

    /*Set PA5 as output pin*/
    GPIOA->MODER |= (1U<<10);
    GPIOA->MODER &= ~(1U<<11);

    /*Set PC13 as input pin*/
    GPIOC->MODER &= ~(1U<<26);
    GPIOC->MODER &= ~(1U<<27);

    while(1)
    {
        /*Check if BTN is pressed*/
        if(GPIOC->IDR & BTN_PIN)
        {
            /*Turn on led*/
            GPIOA->BSRR = LED_PIN;
        }
        else
        {
            /*Turn off led*/
            GPIOA->BSRR = (1U<<21); // Reset PA5
         }
		}
}