150-600mm f/5-6.3 with a 1.4X teleconverter attached makes for an
actual 210-840mm f/7.1-9 combo.
When it's used on an APS-C (1.5X lens factor) D500, it's
equivalent to 315-1260mm on "full frame".
... or it's
equivalent to 670-2680mm on 6x7cm medium format film (0.47X lens factor).
... or it's
equivalent to 157-630mm on micro 4/3 sensor (2x lens factor).
... or it's
equivalent to 1086-4345mm on 4x5" large format film (0.29X lens factor).
... or it's
equivalent to 56-223mm on a tiny 1/2.3" sensor (5.64X lens factor).
Incidentally, in the past "full frame" format (24x36mm) was referred to at times as "miniature" and other times as "double frame".
35mm film width was invented in Thomas Edison's workshop prior to 1890. It was derived from splitting standard 70mm wide film lengthwise (already in wide use in various medium format cameras). The original intent was for cinematic cameras, but the individual image size was 18x24 initially for both movies and still photos. The film was used initially in Edison's Kinetoscope "moving picture" device that was officially introduced in 1893.
It wasn't until around 20 years later that the first cameras were introduced that made what we today call "full frame" 24x36mm format images, using the same 35mm wide film. The Simplex camera of 1914 was the first commercially successful camera to make images that format, using 50 foot rolls of film to take either 800 18x24mm or 400 24x36mm images. Oskar Barnack was inventing the Leica about the same time and brought it to market in 1925, initially using 35mm film in user-reloadable cartridges. The daylight-loading, single-use 35mm film cassette we're all most familiar with was intro'd by Kodak in 1934, primarily for use in Kodak's own Retina cameras but also usable in Leica, Contax and a number of similar cameras that had by then entered the market. Up to and for some time after WWII, 35mm film cameras were often referred to as "miniature" format (medium formats using 70mm wide film or larger formats using sheet film continued in wide use). Cameras making either 18x24mm (sometimes called "single frame", sometimes "half frame") and today's "full frame" 24x26mm (sometimes called "double frame") images on 35mm wide film gained popularity through the 1950s, became the dominant formats in the 1960s and continued to be produced through the 1990s. A few cameras were even able to make either single/half frame or full/double frame format.
I always find it a bit ironic that so many digital sensor formats such as "full frame", "APS-C" and "APS-H" are named after film formats!
As to using a 1.4X on a 150-600mm on a Nikon D500... methinks it would be simpler and better to just get closer to your subject and forget about using the lens with any teleconverter. Seriously! 600mm on a D500 is a whole heck of a lot of lens!