you have to distinguish between console/terminal-window and a shell running within°, and you can run multiple shells nested within each other.²

in your case portableshell.bat starts a fresh cmd° shell as a sub-process inside the current terminal window, after typing exit you'll be back to the outer powershell.

compare

C:\tmp>
C:\tmp>cmd                                        # new sub shell
Microsoft Windows [Version 10.0.19044.2251]
(c) Microsoft Corporation. Alle Rechte vorbehalten.

C:\tmp>exit                                       # leave sub shell

C:\tmp>
[download]

vs

C:\tmp>start cmd                                  # new window with su
+b shell

C:\tmp>
[download]

°) that's mostly *nix speak, not sure how they call these on win

²) the motivation for the name "shell" is a surrounding process managing an environment for sub-processes