Also, the netlist which gets generated for fets... the dummy and main has different

m

.subckt NMOS D G S B l=2.0 w=4.15 m=2 dm=1 
XMAIN   D G S B sky130_fd_pr__nfet_01v8 l={l} w={w} m={m}
XDUMMY1 B B B B sky130_fd_pr__nfet_01v8 l={l} w={w} m={dm}
XDUMMY2 B B B B sky130_fd_pr__nfet_01v8 l={l} w={w} m={dm}
.ends NMOS

It will be helpful if someone explains this... so that we can follow what is being done...

To answer @Subham Pal's question, the the NMOS subckt definition has

m=4

which is its default value. When it is initialized inside the differential pair, the multiplier is set to

m=2

since that's the multiplier for the diff pair.

This has nothing to do with

mtop / 2

which is how the multiplier parameter of the diffpair is defined. It is defined as the total multiplier of both the transistors in the pair, and hence the

m

for each transistor is half of it.