How can I create an alias of a selected file in Finder using Shortcuts?

You're right. There's no built-in shortcut to make an alias, but it's not that hard to do via AppleScript, which can be part of your shortcut.

Here's a quick and dirty Shortcut that takes files via Quick Actions and makes an alias on the desktop.

For ease, the raw AppleScript is just:

on run {input, parameters}
	tell application "Finder"
		repeat with each_item in input
			make new alias to each_item at (path to desktop)
		end repeat
	end tell
end run

If you don't want to make the alias on the Desktop, enter the path instead of '(path to desktop)'. Finding the path can be tricky, but AppleScript's path to command will help... for example, you can simply say path to documents folder or path to home folder for your home directory, etc. or you can type it out using :-delimited path (e.g. "Macintosh HD:Users:username:Documents:My Aliases Folder")

> If I want the alias created to be deposited back into the folder where the original file came from, do you know what command I can use?

Sure. Files have an inherent container property that maps to its folder on disk, so use that:

on run {input, parameters}
	tell application "Finder"
		repeat with each_item in input
			make new alias to each_item at (get container of each_item)
		end repeat
	end tell
end run