Show Form Screen will give you a native form, so you don’t want to use that.
The usual way to create a Custom Form is with a Show New Screen → This item action. (there are other ways, but that’s the most common).
uhh, nah…
Show New Screen gives you a new details screen with a default set of components that you can customise. That screen will be tied to the action that calls it.
If you use Show Form Screen, you’ll be able to customise your Form Screen, but the changes will apply to everywhere else that you use a Form Screen for the same table.
I’d suggest making a copy of my Custom Forms Concept App and study it. I think that should answer most of your questions.