Three consonants carry one idea: going down, settling into something. The sun sinks and you get sunset; a hollow sinks into a wall and you get a socket; particles settle and you get sedimentation; a person sinks into a task and is absorbed in it. The causative is the most telling: to make money sink into a venture is to invest it. Pour the root into the pa'al and a thing sinks on its own; pour it into the hif'il and you sink something, you submerge it, you put it to work. Every word here is verified against pealim, niqqud included.
The same root runs through the verb system. Each binyan is a voice: it bends the one idea into active, passive, causative or reciprocal. Tap to hear.
Two binyanim carry the load: the intransitive pa'al (a thing sinks by itself, the sun sets) and the causative hif'il (you make a thing sink, submerge it, or sink money into a venture, i.e. invest). The shift from "sink" to "invest" is pure causative logic: you cause your capital to go down into the work. Pi'el, pu'al, huf'al and hitpa'el are not lexicalized as everyday verbs for this root.
Same root letters, different noun moulds (mishkalim). The mould tells you the function: who does it, the act of doing it, or the thing produced.
Strip the prefixes, suffixes and vowels. Three consonants remain, and the meaning lives there.