For a Wye connection this is basically what you have.
Think of a 120/208 three phase system as the capital letter 'Y'. From the center to any of the three end points you have 120 units of measure. From any point on the end to another end point you will have 208 units of measure.
Another way to think of it as three triangles with two legs 120 units long and a base 208 units long. The one corner with the two 120 sides is the center of the 'Y'. the other two corners with a 120 and a 208 represent two of the ends of the letter 'Y'.
If you place the three triangles together with all the 120 sides against each other you end up with three equilateral points 120 degrees apart in reference to a single rotation or circle.
The more common high power connection in North America is the 277/480. Its the same ratios just at a higher voltage.
The more common European standard is the 240/416 I think.
Simple geometry using nothing more than triangles and a circle.
For a delta connection (Δ) you just have one equilateral triangle with no center point. They are most often 240 volt or 480 volt. In some applications they add the center tap to one side of the triangle so it has one leg split into a 120/240 or a 240/480 connection. Then you get the 120/208/240 or the 240/416/480 three phase connection system.
The 208 or 416 volts come from that one center tapped side going strait across to the opposite corner.
On a three phase name plate the amps rating is the amps per each leg. So a 240 volt, 20 amp three phase circuit carries the same power as three single phase circuits of equal voltage or 240 volts at 60 amps single phase while only adding one more wire.