You can look at the source code for Appler and read all about the history of it on GitHub: https://github.com/zajo/appler

Now try writing some code for the Apple II!

• Write in BASIC
• Or try assembly!
  • Easy 6502 - tutorial for learning 6502 assembly
  • 6502.org - huge resource for 6502 programming

And of course, archive.org has a huge library of Apple II books, software, and other goodies. Just search for "Apple II" and see what you can find.

Apple II Software:

• https://mirrors.apple2.org.za/ftp.apple.asimov.net/images/
• https://apple2online.com

DOSBox - Don't forget, now that you have DOSBox you can run all kinds of old DOS software.