ESP32 UART buffering of received data

[sid34]

If we look in uart.c, which I think I'm correct in saying lives in the ESP-IDF API layer underlying the Arduino-esp32 API, we find functions like:

    esp_err_t uart_get_buffered_data_len(uart_port_t uart_num, size_t *size)
    {
        ESP_RETURN_ON_FALSE((uart_num < UART_NUM_MAX), ESP_FAIL, UART_TAG, "uart_num error");
        ESP_RETURN_ON_FALSE((p_uart_obj[uart_num]), ESP_FAIL, UART_TAG, "uart driver error");
        UART_ENTER_CRITICAL(&(uart_context[uart_num].spinlock));
        *size = p_uart_obj[uart_num]->rx_buffered_len;
        UART_EXIT_CRITICAL(&(uart_context[uart_num].spinlock));
        return ESP_OK;
    }

Is this function, and the ones that accompany it, accessing a ring buffer or the actual UART FIFO RX buffer?

I've made a number of assumptions in asking this question, and I apologise for my naivety. However, I'd be extremely grateful for clarification on a number of basic matters.

1. Is the UART FIFO buffer a "normal" linear buffer of fixed length? (I believe it is.)
2. Is the UART FIFO memory that exists in the UART chip rather than in the ESP32's RAM or flash memory? (I understand that the ESP is a SoC, so it's all one chip, but...)
3. Does FreeRTOS implement its own ring buffer "on top of" the UART FIFO buffer? (I think it does.)
4. Why have a ring buffer and a UART FIFO buffer in the first place?
5. When does FreeRTOS copy data from the UART FIFO buffer to the ring buffer? Based on what sort of criteria?

Many thanks.