Simplest way to divide up a project / sketch?

[mrburnette]

A different approach to easy BU, just one folder to ZIP

Duplicated libraries - Using Arduino / Programming Questions - Arduino Forum

also
Sketches and main() - Using Arduino / Programming Questions - Arduino Forum

also
Using Modern C++ Techniques With Arduino | Hackaday

also (NO SETUP?MAIN)

Optimizing Arduino Code: no setup(), no loop() ⛔