`emacs` and `Emacs` start the same application?
The default install of macOS uses a case-insensitive file system. This is true of both HFS+ installations of macOS 10.12 (Sierra) and earlier, as well as APFS installation of macOS 10.13 (High Sierra) and later.

The use of a case-insensitive file systems means that files and folders can be reached with any mix of case in the name. The name used to create the file/folder is used verbatim when displaying the name of the file, but when you need to access it, you can mix it up:

$ touch myTeSt
$ ls
myTeSt
$ rm mytest
$ 


I.e. the case is displayed as "myTeSt" when running ls, but you can delete the file using "mytest", "MYTEST" or any variation thereof.

Naturally, this also extends to programs that you can then start using any variation of case inside the program name.

Note that it is possible to install macOS on a case-sensitive file system, where this does not apply, however it is not the default install.