When to use a diffuser, a reflector, a fill flash and an ND filter?

This question stems from the answers I received for another question "Is the 35mm on a cropped sensor good for fashion photography?"

I am watching two videos in which the photographer uses an HSS flash to take pictures and the pictures become incredibly amazing. For example, this youtube video "Creating colors with flash off"

The photographer demonstrates how the images look flat when the images are taken only in natural light, while the photos taken with flash have good shades of color and depth. I wanted to try this approach but I soon realized that my Nikon D5600 (with 50mm 1.4G) did not support the HSS.

I've started looking for workarounds and finding this High Speed ​​Sync video vs ND Filters where the photographer shows that the ND filter and the HSS flash can both give the same results.

In other instructional videos, the photographer simply uses a diffuser and a reflector. But most of the time, I see the reflector and the diffuser held by a person.

I want to understand when to use diffuser, reflector, ND and flash add-on?

Plus, if I use a tachometer on a person's face shadows when the ND filter is turned on, does that replace the diffuser and the reflector?