Q:

Is there a way to reverse engineer this small tool "website2apk" and design my own?

I recently came across this tool and was amazed by its size and how it can build release-apk, without even bothering about Android development environment set up on the machine, neither Cordova nor Android Studio is required.
A bit of research showed me that the what this tool is doing is simple (most related material I found was of 2008-2014). 
Being new in reverse engineering, I wonder if there is a way I can understand its workaround and build a similar tool to automate my Cordova projects.

A:

Depending on what the tool is written in.
If you can it with binary analyser like ProtectionId or Pied it should tell you what compile was used to build the binary.
If it is build using the .NET library then you can use a decompiler like dnSpy or Reflector to view the source code. Java is also reflectable so a tool like JD/JD-GUI would be able to help recover the source code.
If it is a native binary (C/C++) it becomes a little trickier, you will need to use something like hexrays or ghidra to decompile binary and give a C like pseudo code which can be used to rewrite an application.
Hopefully this helps

