What does the inc folder mean?

Question

WordPress

And how is it different from template-parts?
What and where is better to shove?

Answer 1

includes - for example, you can include files with your own functions and other custom code
template-part - it is clear from the name that you need to store template parts here

Answer 2

Depends on the programmer. Usually, as written above - some kind of extra. add up the functionality.