About mosfets being used as switches, i think its a semantics difference of what that means literally. What the use of the words evocate / what expression is understood as. So ill try clarify what means to me. Mosfet has an elementary function of iDS=k/2*(vGS-VT)^2 which is normally a curve on the graph (^2), and *ONLY* when the mosfet is used as a switch (fully on/off) then this function can be approximated by considering the mosfet behavior similar to a resistor, and then instead of all those info, we can just simply use one rDS(on), e.g. resistance from drain to source when its on. So when datasheet proeminently mentions the on-resistance like rds or r(on), it menstions that because that mosfet is used as a switch. Stepper drivers have mosfets in a h-bridge and used as switches, datasheets gives miliohms on-resistance. Same with heater mosfets, and again the case of almost all power applications of mosfets, all around.
The alternative common area use of mosfet is as an amplifier in a small-signal domain, e.g. very low power. But again because its a curve function, its not all that good, that curve implies distorsion. In small signal use, we "zoom in" on a small segment of that curve untill it "appears" to be a line. Outside small signal, for amplification is better to use a regular transisfor which function is simply input*gain=out, because this one has no squared factor and being graphically linear it wont distort the output (so much).
The alternative common area use of mosfet is as an amplifier in a small-signal domain, e.g. very low power. But again because its a curve function, its not all that good, that curve implies distorsion. In small signal use, we "zoom in" on a small segment of that curve untill it "appears" to be a line. Outside small signal, for amplification is better to use a regular transisfor which function is simply input*gain=out, because this one has no squared factor and being graphically linear it wont distort the output (so much).