Help identifying switch and wiring

Our house that we purchased a couple years ago was built in 1960 and I am finally getting around to updating the last of the remaining single pole "flip" type switches with the Decora style. However, I need assistance trying to figure out one particular switch that controls the light for the ceiling fan/light combo. The ceiling fan/light also has two separate pull chains to control either the light or the fan. Attached are 4 images of the switch in question, along with explanations for each.

Here are my questions/concerns:
1. I've never come across thick stranded wiring, with a thick rubber type covering, of the type used for both of these wall switches (one for the light, the adjacent one for the fan). All other wiring in the house is solid copper, either 14 or 12 AWG. Can anyone offer info on the stranded wire used here? It seems thicker than 14 ga, but the circuit is 15 amp.

2. Why is a neutral wire backstabbed into the light switch. There are no other neutral wires located inside either the box for the fan or the switch. Unless the white neutral is being run as a hot, but it is not marked anywhere to indicate so.

All I want to do is replace the original light switch shown in the photos with a Decora switch and need to know the best way to do so considering the way it's currently wired. Unless it won't be possible.

Just trying to get some professional opinions for guidance as I try to figure this one out. Tks.