As mentioned previously, the default Twentytwenty themes are some of the best examples of good theme development.
You can see that the main theme template files are in the root directory, while JavaScript, CSS, images are placed in assets directory, template-parts are placed in under respective subdirectory of template-parts and collection of functions related to core functionalities are placed in inc directory.
At this time there are no required folders within a WordPress theme. However, WordPress does recognize the following folders by default.