What's inside a .dll file? Does it contain a source code of a program?

Answer 1

https://support.microsoft.com/en-us/help/815065/what-is-a-dll

What is a DLL?

A DLL is a library that contains code and data that can be used by more than one program at the same time. For example, in Windows operating systems, the Comdlg32 DLL performs common dialog box related functions. Therefore, each program can use the functionality that is contained in this DLL to implement an Open dialog box. This helps promote code reuse and efficient memory usage.

By using a DLL, a program can be modularized into separate components. For example, an accounting program may be sold by module. Each module can be loaded into the main program at run time if that module is installed. Because the modules are separate, the load time of the program is faster, and a module is only loaded when that functionality is requested.

Additionally, updates are easier to apply to each module without affecting other parts of the program. For example, you may have a payroll program, and the tax rates change each year. When these changes are isolated to a DLL, you can apply an update without needing to build or install the whole program again.

The following list describes some of the files that are implemented as DLLs in Windows operating systems:

ActiveX Controls (.ocx) files
An example of an ActiveX control is a calendar control that lets you select a date from a calendar.

Control Panel (.cpl) files
An example of a .cpl file is an item that is located in Control Panel. Each item is a specialized DLL.

Device driver (.drv) files
An example of a device driver is a printer driver that controls the printing to a printer.

Answer 2

No source code:

Here's Microsoft on DLLs:
https://support.microsoft.com/en-gb/help/815065/what-is-a-dll