What are the options available to have an external output to a monitor from a laptop running vMix with a single HDMI camera input?

On Laptops you can easy access the „Fullscreen“ Displays. This are Display Outputs rendered on the Graphics card.
Vmix Can distinguish four Displays. The Number may be sorted an other way than windows does.
The FullScreen Tab is above the middle controls.
You can Route PRV, PGM or the SwitcherMV on these Outputs as well as any Vmix Input (Also InputMVs)

So a typical Usage is to show a Video Loop on a screen behind the talent or show a caller on Display wich is filmed by an camera input and going to program.

Spoken in video switchers these are flexible aux bus outputs.

Dont use the word external. External Outputs are a special feauture in vMix. These are Outputs rendered on SDI cards from AJA, BMD or Blufish444.
The externals are connected to the Videobusses 1&2
1 is also linked to the Recorder and the Streamer in Vmix. If you Stream this should be used with PGM Signal and the external use is to send the Programm to external sdi monitors with waveform or for lighthing and director.

External 2 is linked to video bus 2 wich usable as an aux on SDI.