120<0? and 240<120? are just shorthand. Consider the following
120<0? = sqrt(2)*120*cos(ωt+0?) = sqrt(2)*120*cos(ωt+0?*pi()/180?)
240<120? = sqrt(2)*240*cos(ωt+120?) = sqrt(2)*240*cos(ωt+120?*pi()/180?)
where ω = 2*Pi*freq = 377
At t=0:
120<0? = sqrt(2)*120*cos(377*0+0?*pi()/180?) = 169.71
240<120? = sqrt(2)*240*cos(377*0+120?*pi()/180?) = -169.71
The sum equals zero.
You can do this as time continues on. Here are the results for t = 0 to 0.0175 sec:
Time=0.000000 sec, 169.71 + -169.71 = 0.00
Time=0.000833 sec, 161.40 + -252.23 = -90.83
Time=0.001667 sec, 137.29 + -310.07 = -172.78
Time=0.002500 sec, 99.75 + -337.55 = -237.81
Time=0.003333 sec, 52.44 + -331.99 = -279.56
Time=0.004167 sec, -0.01 + -293.93 = -293.94
Time=0.005000 sec, -52.45 + -227.10 = -279.55
Time=0.005833 sec, -99.76 + -138.03 = -237.79
Time=0.006667 sec, -137.30 + -35.46 = -172.76
Time=0.007500 sec, -161.40 + 70.59 = -90.81
Time=0.008333 sec, -169.71 + 169.73 = 0.02
Time=0.009167 sec, -161.40 + 252.25 = 90.85
Time=0.010000 sec, -137.29 + 310.08 = 172.79
Time=0.010833 sec, -99.74 + 337.56 = 237.82
Time=0.011667 sec, -52.43 + 331.99 = 279.56
Time=0.012500 sec, 0.02 + 293.92 = 293.94
Time=0.013333 sec, 52.46 + 227.08 = 279.54
Time=0.014167 sec, 99.77 + 138.01 = 237.78
Time=0.015000 sec, 137.31 + 35.43 = 172.74
Time=0.015833 sec, 161.41 + -70.61 = 90.79
Time=0.016667 sec, 169.71 + -169.75 = -0.04
Time=0.017500 sec, 161.39 + -252.27 = -90.88
So we can see that in real time, the voltage is a summation. The shorthand version uses vector addition so we can work with the rms values so we can see what happens on average, not just at one instant in time.