To turn on a MOSFET the gate voltage must be higher than the source node, positive gate-to-source voltage for an N-MOSFET, and negative gate-to-source voltage for a P-MOSFET. Thus the type you use depends upon the polarity of the signal you are switching and the drive signal. In this case the voltage and drive signals are +5V. Thus for the FETs that control the +5V (M33, M37) you use P-MOSFETs which can be turned on by connecting their gates to 0V and turned off by connecting their gates to +5V (since their source nodes are at +5V). Similarly for the FETs that switch the voltages to ground (M35, M38) you use an N-MOSFET which can be turned on by connection their gates to +5V and turned off by connecting their gates to 0V (since their source nodes are at 0V).