Apple provides a list of directories here: http://support.apple.com/kb/HT2435

User ~/Library/Fonts/ Each user has complete control over the fonts installed in their Home. These fonts are available to that user when he or she is logged in to the computer. Fonts installed here are not available to all users of the computer.

Local /Library/Fonts/ Any local user of the computer can use fonts installed in this folder. Mac OS X does not require these additional fonts for system operation. An admin user can modify the contents of this folder. This is the recommended location for fonts that are shared among applications.

Network /Network/Library/Fonts/ The Network folder is for fonts shared among all users of a local area network. This feature is normally used on network file servers, under the control of a network administrator.

System /System/Library/Fonts/ Mac OS X requires fonts in this folder for system use and displays. They should not be manually altered or removed.

Classic /System Folder/Fonts/ This folder contains fonts used by the Classic environment (Mac OS X v10.4 or earlier only). If more than one Mac OS 9.1 System Folder is present, only fonts in the System Folder selected in the Classic pane of System Preferences are used. Classic applications can access only these fonts, not those stored elsewhere. Conversely, Mac OS X applications can use these fonts, even when the Classic environment is not active.

Yes, its located at System/Library/Fonts. There are several versions for different uses. SF-Pro for macs, and iPads, SF Compact for iWatch. And a third one for the terminal called SF-mono

Apple has a very restrictive license for these fonts. It may be a good idea to read it.

To find and install default Microsoft fonts on a Mac:

1. Navigate HERE in your browser.
2. Download the .ZIP file.
3. Unzip it (double-click it).
4. Open the folder that appears.
5. Select all the .TTF files inside the folder.
6. With the files selected, right-click.
7. From the pop-up menu, choose "open with → Font Book".
8. Click "Install Font" for each font.

Note: Changes to fonts take effect when an application is opened or a user logs in to the account or computer on which the changes occurred, see support.apple.com.