How does a calculator convert pure binary to BCD?

If you are implementing this with hard-wired gates, as opposed to computer code, you are going to have a lot of gates, even for 2 digits, let alone 8. Starting with your binary encoded number, you need to do a division by 10, using binary arithmetic. The binary remainder forms the least significant (rightmost) digit and the binary quotient (the result of the division sum) becomes the next digit. You will then have a set of bits to represent each digit. At that point you could use a chip like the 4511 BCD to 7-segment display driver. Good luck.