There are two ways to get a new floor past a door casing. You can cut the floor around the shape of the casing, or you can cut the casing and slide the floor underneath. The second is faster, more forgiving and looks better, and the only reason it is not universal is that it has to be done before the floor is laid, when the temptation is to get boards down and worry about details later.
Timber takes up and gives off moisture until it is in balance with the air around it, and as it does so it changes size across the grain. Every problem that shows up in a wood floor months after it was laid is a version of this one fact. A floor laid dry in a damp room swells and buckles; a floor laid damp in a dry room shrinks and opens gaps between every board.
A floor squeak is movement, and the movement is almost always a board rising off its support under load and dropping back as the load passes. The noise itself is a nail shank rubbing in its hole, or two edges of timber moving against each other. Find which of those two it is and the repair is straightforward. Guess, and you will fix half the squeaks in the room and create a few new ones.