Powershell is Open Source
that is a thing that started under Nadella's leadership, during the 90s it was a different story.
Those two "PowerShell"s are not the same. For the sake of cross platform deployment, they moved away from the original a lot -though they managed to support many things in time. The old and original one was released in 2006,IIRC, so it didn't exist in 90s.
There are a multiple publically accessible/usable sources which can be found via search engine(s) to extend bash / unix shell functionality based on points discussed in topic link.
few article link topic snippits:
* approaches to autocompletion : [0][1]
* 'man' command and/or cli option -h to cli command shows 'docstring' / options
* jq resources for shell plugin[2]
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[0] https://faun.pub/configure-bash-auto-completion-tab-completi...
[1] https://tiswww.case.edu/php/chet/readline/rltop.html
[2] https://github.com/fiatjaf/awesome-jq