What do all site collections and sites within a web application share in Sharepoint?

Yes, multiple content types can indeed be added to a document library in SharePoint. This feature allows site users to use the New Document command to create specialized content quickly. To add multiple content types to a list or library, you must have at least the Design permission level for that list or library. You also need to have multiple content types enabled. The steps to add a content type to a list or library are as follows:

• Go to the settings for the list or library.
• Under Content Types, select Add from existing site content types.
• If Content Types doesn't appear, select Advanced settings, and select Yes under Allow management of content types?, and then select OK.
• In the Available Site Content Types list, select the name of the content type that you want, and to move the selected content type to the Content types to add list, select Add.
• To add more content types, repeat steps 4 and 5.
• When you finish selecting all of the content types that you want to add, select OK.

Please note that if the list or library isn't set up to allow multiple content types, the Content Types section does not appear on the Settings page for the list or library. To learn how to allow multiple content types, see Turn on support for multiple content types in a list or library.

Sharepoint 2013 workflows run every 5 minutes in the timer job by default.

Site collection, site, and list/library are the three levels where security handled in SharePoint.

In SharePoint, you can create several types of lists based on your specific needs like Communication Lists, Tracking Lists and Custom Lists

A site collection is used to provide a grouping of sharepoint sites