If a transistor is a current sink you expect infinite impedance output, but due to the Early Effect shunt resistance and output capacitance and layout stray impedances including probing method, this can be >1M and <1pF depending on size , ratings and frequency applied.
Using a short circuit current with Rc=0 is the easiest way uA to Amps but then Vce= Vcc may not be your operating condition or temperature rise due to Ic*Vce *Rja = temperature rise above 25’C, which affects the results.
Leakage current, increases as well as hFE(dc), Hfe(ac) and GBW (MHz) with temperature as Vbe reduces.
This can be measured with a micro amp to Amp range of collector currents compared with input currents in either Vdc or Vac at some frequency on a DC bias.
Then z21 is the ratio of currents for v21/i21=z21. But v2 the collector is fixed Vcc and v1 is your incremental Vbe bias voltage and same for i1 is the incremental bias current.
You know Ic is a function of Vbe, but for the AC model you are only interested in the incremental or small signal impedance gain (transfer function) at some Q operating point of Ic(dc) with a swept frequency for the s or z(f) domain transfer function over the range of interest which may include near DC.