ESP32 Forum

I'm familiar with SPI bus from other projects and wikipedia, it usually has four signals, SPI_MOSI, SPI_MISO, SPI_CLK, and SPI_CS
however from the datasheet I've only found the VSPICLK and VSPICS0.
I've kinda gleamed that
VSPIQ is SPI_MISO nad
VSPID is SPI_MOSI.

but what is VSPIHD?

It's 'Hold' which is an input on most common SPI flash chips. (You may also run into 'WP' which is for Write Protect, which is also an input on chips like this.) SPI normally uses them if you use quad SPI mode, in which the SPI port sends/receives 4 bits at a time. You;d also need a peripheral which supports that, obviously.