You can get really far just by using macros, so don't overlook that, but the language is called VBA and its documentation is here: https://docs.microsoft.com/en-us/office/vba/api/overview/

The documentation may look daunting, but VBA isn't that difficult to do useful things with. You'll be mostly copying/pasting from google like most professional coders :)

You can start with a macro and turn it into VBA, which is sometimes useful.