Where can I see the full key-to-character mapping for a language-specific keyboard? I'd like to know which character is output from which key, including with modifiers like  ⌥ ⌘  ⇧ ⌃, including the character and its Unicode number.
Only the primary characters appear on my physical keyboard, but some less-known ones are produced with the modifiers.
This can actually be seen under System Preferences-&gt;Keyboard-&gt;Input Sources; Select language and then press modifier keys. However, I cannot screenshot the image of the keyboard with a modifier because the screenshot key-combination overrides the modifier key; and in any case this  is a tiny  image of the keyboard, whereas I would like a precise listing in text form.
Is this in a configuration file somewhere on my Macbook (2017 15 inch; OS 10.15.7), or can I output it with a certain command?

[Edit]: In addition to the answer below, using Ukelele, note this  code which processes Ukelele output into a searcable document.
Use Keyboard Viewer instead of the preferences pane.
Use the Screenshot app found in Applications/Utilities to create shots of the different modifier levels.
I don't know of any easy way to get the Unicode numbers.  Perhaps the Ukelele app, which can make a copy of the current input source for analysis.