The one into the big filter is the overall inlet of raw water. The black one is finished RO water (the small cylinder is probably a remineralizer). There will also be a waste outlet somewhere on the other side of the machine.
black is usually the color for the waste water (brine) to sink drain
but in this case it isprobably after the finishing carbon filter(smaller white cylinder)
therefore the filtered water output
need more/better pics.
These types of systems usually have a 3gal undersink pressure tank.
Need more pics.
The inlet on the side of the big filter is where the unfiltered water source is routed to.
Typically the first big filter is sediment, then 2 carbon filters.
I might know a little about these (I've installed half a dozen)