All of the technical regulations are set in advance of the season (this year’s is https://www.fia.com/file/78015/download/26184). The teams are only allowed to test their cars during a fixed number of “test days”, so they do a _ton_ of simulation pretty much all the time (https://arstechnica.com/cars/2017/04/formula-1-technology/).
The chassis (the tub that the driver & engine sit in) doesn’t change much over a season, so the time will be occupied by the team finding ways to move and re-shape things like front and rear wings, so as to increase downforce.
Downforce is king because it allows you to make sharper turns at higher speed without understeeting (where the front refuses to turn) or oversteering (where the back refuses to stop turning). And the technical regulations are really finicky about many areas of the car. And in many ways, the three “free practice” sessions before qualifying are as much about in-season development as they are about getting used to the track.
Yes, as drivers get time in the real car, they begin to understand where they can push. But also the engineers are seeing how the car is responding during real-world conditions, to feed that into the simulation.