Can someone explain how internet bandwidth is distributed in an office environment?

Hello,

If my office (80 users total) purchased 300 mbps of data from our ISP, why is it that when I check each users network adapter status, on wi-fi it shows 400mbps and on ethernet, it shows 1000mbps. Are those numbers just showing theoretically what that device could potentially get if available?

And when I run a speed test on some users computer, it only shows about 120mbps.

I think I understand bandwidth in the sense of the number of lanes on a highway but all these numbers don't make sense to me.

And I've heard ISP and other IT people mention..."You could get up to speeds of blah blah...." Is there not a minimum or maximum guarantee of the amount of data we're paying for? Is it just what's available that day or hour in that area?

Could someone explain it to me, thank!