Hi,
You can read that : http://build.prestashop.com/news/prestashop-in-2019-and-beyond-part-1-current-architecture/
And others articles on it about development.
Moreover, you can read the devdoc here : https://devdocs.prestashop.com/1.7/basics/introduction/
You can find examples of module for 1.7 here : https://github.com/friends-of-prestashop (more to come)
You can look on some native modules refactoring, with some tests and others great dev tools :
- FacetedSearch : https://github.com/PrestaShop/ps_facetedsearch/pull/52 and others Pull Requests
- ProductComments : https://github.com/PrestaShop/productcomments/pull/26 and others Pull Requests
You can search more modules and others tools on PrestaShop list of Repositories on GitHub https://github.com/PrestaShop (sort Repositories of Prestahop by recently updated)
You can read latest Pull Requests on PrestaShop Repository to discover others cool things incoming. https://github.com/PrestaShop/PrestaShop/pulls
Codings Standard of PrestaShop for your modules : https://github.com/PrestaShop/php-coding-standards
and more few things...