Inside the computer every number is stored in binary (base 2), using only 0 and 1. Each place is worth double the one to its right, 1, 2, 4, 8, 16 and so on, so 1101 is 8 + 4 + 0 + 1, which is 13.

Hexadecimal (base 16) uses 0 to 9 and then a to f for 10 to 15. Each hex digit stands for exactly four binary digits, which is why hex is used for colours, memory addresses and looking inside files.

n = 202
print(bin(n), hex(n))
print(f"{n:b} {n:08b} {n:x}")
print(int("11001010", 2), int("ca", 16))
print(0b1101, 0xff)

bin() and hex() add a 0b or 0x prefix to show the base. Format specs (:b, :08b, :x) leave it off, and 08 pads to eight digits with zeros. int(text, base) goes the other way.

To work out binary yourself, keep dividing by 2. The remainders, read from last to first, are the binary digits:

n = 13
digits = ""
while n > 0:
    digits = str(n % 2) + digits
    n = n // 2
print(digits)