I'm kind of confused about digital transmission.
Is the bandwidth occupied by some digital signal the Fourier spectrum of the bit (pulse) format,or the one determined by the bit rate?
Bandwidth is of course occupied by the signal itself. The modulation design of the signal determines the bandwidth. That's why pulse shaping techniques are adopted by many different kind of modulation techniques.
However, our bandwidth we occupy has a DIRECT relation with our maximum bit rate. In Shannon's theorem:
$$ C = B * log_2(1 + S/N) $$
This theorem tells us that we CANNOT transmit more bits per second through that channel in good condition.
Hence, our signal determines our bandwidth, and our bandwidth determines our maximum achievable bit rate.
It depends on some number of things like Inter Symbol Interference and Nyquist Theorem. It's relatively a long topic to explain each element. Please take a look at the terms I have mentioned (Nyquist and Inter-Symbol Interference) and if there is a spot if you have a hard time to understand, please don't hesitate to ask.
As always, Have a nice day.