System modules (both standard and downloaded from Bitrix24 Marketplace) are located in the bitrix/modules system folder. User modules may be located in the folder /local/modules. Such division allows for third party developers to conveniently organize the version control for their implementations by maintaining a standard product update system.
Module files and folder structure for core D7 has remained the same as in the old version of the core.
Module API (classes, logic) in a new core is located in subfolder /lib of the module folder.
For example, the path to the Main module is as follows:
bitrix/modules/main/lib. The same folder is used to connect API when corresponding rules are met.
Note: This folder does not have the required status, if your class does not have its own methods.