Is the human voice considered polyphonic?

It is considered monophonic. To answer your other question, what makes a chord depends on all the notes being played at a given time by all the sounding instruments. For example, a bass guitar play an A, A guitar playing an A and an E and a singer singing a C at or around the same time will make an Am chord because the notes A, C, and E are sounding.

Before I move on, Though it is ideal you don't need all 3 notes sounding in a chord to make it a chord. You just need the root and the 3rd sounding. You can exclude the 5th.

If only one monophonic instrument is playing, you could just play arpeggios to imply the chords, but that isn't always the case. When musicians have to harmonize a certain melody, we look at the function of the notes and surrounding notes to determine what chord we put there. This is more complex than what I can explain on here. I will use the key C major to explain. Simply put, if the notes are focused on the 1st or 3rd scale degree, a tonic function can be assumed. A C major chord can be implied. If the focus and function is based around the 5th or 7th, and occasionally the 2nd and scale degree (depending on the next note), a dominant function is assumed and a G or B dim chord can be implied. If the focus is put around the 2nd, 4th, or 6th, a subdominant function is assumed and a Dm, F, or Am chord can be implied.

I’m sorry if this is confusing, but you may have to do your own homework to completely understand this. It will take a college music student about a year to begin grasping this concept.

Thanks. I know the difference between mono and polyphonic, but I still don't understand how we can produce a minor chord vocally. As you point out, the best we should be able to do is sing a minor triad as an arpeggio, and yet we sing in minors all the time without ever singing any actual chords.

No.
It's only capable of one note _at a time_ so monophonic.

A flute can play many different notes, but only one at a time; monophonic.
An organ or piano can play multiple notes simultaneously - that's polyphonic.

https://en.wikipedia.org/wiki/Polyphony_and_monophony_in_instruments