1) Yes, there is a circular buffer. It is done within the "UART" component in the schematics file (Top of the project file tree on the left side of the editor, the file called "TopDesign" should contain the hardware components used in the program. Double-click on the "UART" component, and it should open up with a configuration, showing the TX buffer size which is a circular buffer).
2) The component pushes data to the buffer through hardware interrupts, and reads them out using hardware interrupts, with software just handling the circular buffers (both read and write).
3) I assume by "Ring buffer" that it is the same thing as "Circular buffer" (according to wikipedia). There should be multiple examples within the PSOC creator if you click file->new->project, choose your chip/kit/board, then select code example and pick either UART RX or UART TX to see the example project.
Here is an application example with UART incorporated, but it might be for a different chip.