Some complex questions there… 🙂
The wiki is not complete documentation as far as explaining exactly how the sampling & line multiplication occurs, but at least it describes each option in the menu to some extent.
If you want to see some more info on the default video modes, see the bottom of this github file:
The file is part of the OSSC firmware which describes the behaviour of the tvp7002 video chip (this chip handles the digitization of incoming analog video).
I will also copy paste something I wrote in another thread, regarding the video mode chosen by the OSSC.
The OSSC chooses a “video mode” automatically based on the input it receives and vertical multiplication mode selected, but it doesn’t matter which one it is because you can still freely change the parameters in Advanced Timing, just go into the one that is preselected for the console (it will be there when you select Sampling Opt -> Advanced Timing). Modes that end with L or LB are 288p modes also.
To clarify with your example what you see when choosing Lx3 16:9 mode: 1280×288, means that OSSC identifies the signal as putative 288p because it has 312 lines, and 1280 signifies it is set to Lx3 16:9 (or Lx4, which uses the same horizontal sampling), where the output horizontal H.active width is 1280.
What is important to remember is that the video output is actually extremely flexible in some regards (horizontal resolutions, active video area), while absolutely non-flexible in others (line count and vertical refresh rate). So one can achieve almost any custom resolution, at least as far as the area of active video described, however since total line counts and refresh rate cannot be manipulated, the output will always be more-or-less non-standard.
Another complicating factor is that for some monitors the total horizontal pixels (i.e. samplerate on OSSC) is used to calculate the appropriate display mode (disregarding settings to H.Active), while for others H.Active is more important. This can affect the representation of aspect ratio.
Without going into details, the default H.samplerate used in various modes by OSSC have been selected for best compatibility and preservation of aspect ratio in most scenarios.