Need a more detailed Physics documentation

Based on what I have observed, when force is applied to a rigidbody, it won’t actually be applied until the next physics step. This actually makes a lot of sense since it requires physics simulation to occur only once per physics step rather than for every magnitude of force applied.

So what I need now is official documentation that confirms/rejects my observation and/or verification from a Unity dev. Can anyone provide me with a link to exactly that?

You are pretty much on the money. The physics in Unity is based on top of PhysX. The Developer section of the nVidia site will have more detailed information about how PhysX works.