Why is input resistance parallel with the base of BJT?
That Ri3 is how Q5 sees Q7 and what's connected to it. Ri3 the other end grounded presents for Q5 all what's to the right from Q5 when one calculates only how weak AC components of voltages and currents behave.
As already said in a comment the DC supply is considered as zero ohm wire when one calculates  AC behaviour. DC source has certain constant voltage, no current variations change it, so its like a wire for AC.