Generally speaking—due to constraints of time and fuel costs—both passenger ships and commercial aircraft follow what's known as a "great circle" over the surface of the spherical planet. This is the shortest distance between two points on a sphere.
A great circle path is actually part of the line drawn around the maximum circumference of the planet, and known as an "arc".
And a ship or a plane follows a constant bearing along that arc, although obviously
it crosses each meridian at a different physical angle: no turning left and no turning right. Many flat earthers will tell you erroneously that the ship or plane is
physically "turning slightly" all the time.
Rhumb line and Great Circle in Mercator projection
Note that this diagram is indicating a great circle path on a flat projection, and
not a sphere, which is why the rhumb line
looks straight.