Direct coupling Biasing of Two stage amp
You're correct in spotting that that biasing M8 is indeed "tricky".

To bias M8 at the current provided by M7 the only real option is to use feedback.

I can for example use this circuit as a buffer (1x amplifier) by connecting the Vi- input directly to the output Vo.

Let's check that that is indeed negative feedback: if Vi- goes up in voltage, Id(M1) increases, Id(M4) increases so the base of M8 is pulled up and M8 will push less current into the output so Vo will drop. This drop of Vo counteracts against the rise at Vi- (with which I started) so indeed, this is negative feedback.

The feedback will force the circuit in such a state that the Vgs of M8 is such that Vo will follow Vi+.

Without feedback it is "impossible" to control M8 properly, there is so much gain at the gate of M8 that Vgs of M8 is either large or near zero. Then basically the circuit acts as a comparator.