You can share the widgets, layouts, connectors, modules, and apps you have built in Mendix platform.
By clicking Add Content button in top ribbon of marketplace you will be navigated to the below screen where you can upload your content.
Once you submitted your content successfully, then it will take up to 5 working days for reviewing purpose by Mendix team.
Below doc will help you further –
You should be able to find everything you need here!
Hope it helps!