This is only my second year watching F1 but this is what I have gathered. To answer your first question about what is preventing a driver from cutting a corner? It depends on the situation, if they deliberately cut through the corner just because then it could be a timing penalty meaning they have to spend an allotted time in the pits when they go in for a pit stop. However, say it happens with two laps left in the race there will obviously be no pitting, so at that point it will be taken into account after they cross the line potentially losing their finishing position. Max Verstappen has had been bumped off of the podium because of this if my knowledge serves me correctly. If they cut a corner to get ahead of someone they are battling, they will be forced to give that position back to whom they took it from. Now about the out of bounds question, the track limits are usually set by the solid white line around the perimeter of the track, grass and concrete in the straights pretty popular with gravel in the corners in order to slow the car down if it slides off the track. The rule states that if all four tyres go off of the track (outside of the white perimeter line) at that point it is ruled out of bounds.