Not everything the FE310 chip can do is accessible on the Hifive1B board, just due to lack of space in the Arduino form factor. You have to examine the schematic diagram carefully, as well as the pinout picture at the end of the Getting Started document. Here are some things I have found:
There are two UARTs in the FE310, and both are available as board pin headers, but only UART0 is connected through the JLink chip to the USB, where it appears as /dev/ttyACM0.
There are three SPI controllers, but SPI0 is dedicated to interfacing to the Flash memory, and SPI2 is not connected to anything at all. SPI1 is available on the pin headers and is also directly wired to the ESP32. Each SPI controller can theoretically control 4 chip-select lines, but only 3 are actually available on SPI1, and one of those is dedicated to the ESP32.
The JLink chip seems to be quite complicated, and it is three times the physical size of the actual FE310 CPU!