Is it possible to see the product used SAIL Interfaces?

I see what you mean.
The Appian UI (the admin console, the /design interface) are themselves SAIL interfaces.
So - when you are importing an Appian application - the import box is a SAIL upload field. The buttons are normal SAIL buttons (a!buttonLayout)
Once uploaded - Appian analyzes the package - shows the inspect/import log etc.. behind the scenes Appian's logic does it's magic.

These SAIL interfaces are something which are proprietary to Appian. Appian does not expose those SAIL interfaces.
There is certainly a way to find the names of those SAIL interfaces and even call those SAIL interfaces from normal SAIL designer. (I am not disclosing it)
But - there is no known way(at-least nothing straight-forward) to see the SAIL code of those interfaces.
Just remember those are "system/internal SAIL interfaces" - meant to be used only by the Appian product engineering and they will change in each version.
We as developers are using those SAIL interfaces.
We are users of this platform - and we are not allowed/expected to see the code for the "system/internal SAIL interfaces". Those interfaces are the intellectual property of Appian corporation.

You can certainly build your own product/IP on top of Appian.

But - that does not imply the right to dive into core Appian product code.  Think of another analogy - many people in the world use Microsoft Windows - people run their businesses, do their jobs, create documents , presentations etc. Microsoft is enabling the users. However - this does not mean that you are allowed to reverse engineer Microsoft office/Windows OS - that would be a violation.

Thanks for the detailed reply. my intention is not to miss utilize or reverse engineering. just want to see how the product dev people wrote the logic and what kind of patterns they are using while handling some complicated scenarios.

You can refer the following
1. Appian documentation on sail rules and expressions
2. design patterns in SAIL (in Appian playbook)
3. Sample application which Appian provides (Case management app)
4. Appian applications on AppMarket
5. Best practices related documentation.

The above are plenty of sources to look for good SAIL code - which will give you idea of what design patterns to follow.
You do not need the Appian product SAIL implementation since above sources are most appropriate for studying the best practices/design patterns to follow.

Hi ramp

You will be best served to focus on the Interface recipes that are found in the product documentation that corresponds to your current version at
docs.appian.com/.../SAIL_Recipes.html or
docs.appian.com/.../SAIL_Recipes.html or
docs.appian.com/.../SAIL_Recipes.html , etc.

This material, as well as the input from our colleagues here at community is what will be most constructive as basis for established design patterns.

As others have commented on this thread, it's not appropriate, and may not be ineffective for you to model your own designs based on the code that drives core product interfaces.

Hi ramp

You will be best served to focus on the Interface recipes that are found in the product documentation that corresponds to your current version at
docs.appian.com/.../SAIL_Recipes.html or
docs.appian.com/.../SAIL_Recipes.html or
docs.appian.com/.../SAIL_Recipes.html , etc.

This material, as well as the input from our colleagues here at community is what will be most constructive as basis for established design patterns.

As others have commented on this thread, it's not appropriate, and may not be ineffective for you to model your own designs based on the code that drives core product interfaces.