Question regarding scaling

[Selur]

If you resize in Crop/Resize tab to 1280x720 without using "Auto adjust", and then go to Filtering > Vapoursynth > Frame > Resize and use NNEDI3, will it automatically choose nnedi3_rpow 2x scaling, and then downscale?

yes

And if I want 1920x1080, would it scale 4x by itself and then downscale?

Yes.

You can check the script view to see what Hybrid does,....

To be sure to make this clear: 'Auto adjust' has nothing to do whether down scaling takes place or not.
It either adjusts the width or height so that you do not introduce a distortion during resizing. (while taking input and output PAR into account)
After this, you got a 'Resize Resolution'.

If you disable "Auto Adjust" and change the resolution manually, the changes are high that you are making a mistake.

(Simplified) During resizing with NNEDI3, Hybrid does the following:

• it looks at the current resolution of the source
  
• it determines the factor that is required so archive the desired target resolution (only 2, 4, 8, 16,., can be factors)
  
• it uses NNEDI3
  
• it looks at the current resolution
  
• if that resolution isn't the desired 'Resize Resolution' it resizes to that resolution
  

Unless you do some academic testing, you should never disable 'Auto adjust'.
If the input PAR of your input isn't properly flagged, adjust the input PAR.
The output PAR should be what you require, this PAR will be signaled to output container.
Normally resolutions like 1280x720, 1920x1080 and similar are expected to have a PAR of 1:1.

=> keep "Auto Adjust" turned on

Cu Selur