It sounds like you're talking about the simplified chord fingering which some keyboards offer for auto-accompaniments (aka rhythms/styles). For example, if you press just a C then the auto-accompaniment usually plays a C major chord. The same usually works for any other individual note.
Beyond that, the fingering is completely at the discretion of the manufacturer, and can vary widely from one keyboard to the next. There is no standard that I'm aware of. More importantly, it sometimes bears little or no resemblance to real chords, so there may be no way to make the technique transferrable to a piano or other keyboards.
You will need to check the manual of your keyboard for details, or just experiment and see what happens. If you don't have a hardcopy of the manual anymore then you may be able to find a copy of it or a tutorial online somewhere.
It's worth noting that some keyboards (usually the more expensive ones) offer more than one fingering mode. There may be a 'realistic', 'multi-finger', or 'full-keyboard' mode which is more comparable to real chord voicings.