macOS Plus at the bottom of tableviews, is that standard?

It's just another one of Apple's non-standard standard controls 😉. If you search around there are third-party classes for handling this.

Apple has a bunch of different style they use, some of which break their own interface guidelines. However, in general plus and usually minus buttons are pretty standard to find underneath tables, I think common enough that most people now know it means "add a new item of the type found in this table" (a row), or "delete the selected items in the table". Sometimes you will find a plus button that adds existing items, for example opens up a dialog for you to choose files to add.