Circuit transformation explanation
It looks like the slide 18-14 is missing essential details added to previous slide 18-13.

Allow me to speculate on behalf of authors;


Add  Vin with series \$R_I=5k\$ to base  , convert to Norton equiv.
Add Vout to collector ac-coupled (large C) with load \$R_L=5k\$


Since \$R_F \$ connects to both base and collector, this feedback is modeled as shown for Zin, Zout and Av.

Note that \$R_L\$ must be > \$R_C\$ when AC coupled.  ( I think  someone just forgot to include the schematic)



The value of Rf reduces the voltage gain by opposing the source current so it tends to follow Rf/Ri ratio rather than bias current and hFE.    

Since Rf is opposing the input voltage by the ratio Ri/Rf, it also reduces the voltage gain to this ratio and it lowers the input impedance to Ri+Rf/β and it also shunts the output resistance with Rf.  So this is modeled by shunting Rf to both input and output.



Now hopefully that makes sense.