Hi there
the current I1 will be determined by the biasing circuit, in this case a current mirror, (not shown) that gives a voltage to VBIAS node
On the other hand, the biasing circuit that sets VBIAS2 determines I1+I2.
This means that 400uA will be the value of your ground current, and half of it will flow through M9 and M10 respectively.
The amount of current you give in your differential pair will play a role in the gain and BW of your amplifier.
I hope this helps