Comment by homebrewer

With fish, if the program you're interested in hasn't betrayed the decades-old tradition of shipping man pages, it's often as simple as running `fish_update_completions`.

It parses all man pages on your system and generates completion files for you. By default, they go into ~/.cache/fish/generated_completions/*

If the man page was written poorly/is missing, you can always write your own completion (and hopefully send it upstream). fish uses such a simple format that I don't think there's any need for tutorials save the official doc:

https://fishshell.com/docs/current/completions.html

For example, here's an excerpt from curl

  complete --command curl --short-option 'L' --long-option 'location' --description 'Follow redirects' 
  complete --command curl --short-option 'O' --long-option 'remote-name' --description 'Write output to file named as remote file'