Since you seem to have a semi-infinite series of the
most basic questions I think a textbook is right for
you.
But the textbook isn't going to teach you to
recognize, for example, that "Vb" in the lower is the
exact same thing as M3:M4 common gate node in
the upper, only difference being whether or not the
biasing is shown and how complicated it may be.
The only way to develop this faculty is to think for
yourself, not pose point-questions (that miss the
point) and walk away with point-answers rather than
developing a gut understanding of how circuits work.
CMFB circuit is not to "improve" the current source.
It's to get the right answer (of Vout swinging equally
across Vin null) by setting the current source value.