Hi Sanjay,
1.Marketing email storage:
If you mean marketing email itself, then all of these records are saved as msdyncrm_marketingemail entity,

you could make customization for it as well as other CRM entities,
also Web API CRUD request or client API with formContext/Xrm.
If you mean where the sent marketing email store, there is a link (View in browser/Open it in browser) at upper right corner on some marketing email templates,
then you will find there are stored in http://xxx.marketingusercontent.com
2. You could find all supported API in link below:
https://docs.microsoft.com/en-us/dynamics365/marketing/developer/marketing-developer-guide

All involved entities in a customer journey are supported with API,
Customer journey has its own SDK and segment is similar to common Web API request;
and you can retrieve contact interactions(*) during customer journey.
It seems that marketing email entity doesn't have particular API compared with entities or data(*) above.
There two type data in marketing application:
- Profile data (could be viewed as general Common Data Service entities)
- (*)Interaction data: such as email opens, email clicks, event registrations, page submissions (data in Insights tab)
The later one are not directly accessible compared with profile data,
you need use the Power BI "Azure Blob Storage" connector for this type of data.
https://docs.microsoft.com/en-us/dynamics365/marketing/custom-analytics#data-sources-for-custom-analytics
Regards,
Clofly