Meagan opens her car door and gets in.
Meagan macht die Wagentür auf und steigt ein.
Can I also use
öffnen in this context? What's the difference between aufmachen and öffnen?
It's just a matter of style; the meaning is the same.
In everyday spoken German you say "aufmachen", in written or higher-register German you say or write "öffnen".
There's a slight difference in register: öffnen is considered the standard expression for open, whereas aufmachen is somewhat more informal. It can be used sometimes, but not always, instead of öffnen. You're on the safe side with öffnen.
Just to add to the other answers:
"Öffnen" can be replaced by "aufmachen" only if used transitively; if used reflexively, the replacement is not possible: You can say "die Tür öffnet sich" but not "die Tür macht sich auf".
However note: "Der Laden öffnet um zehn Uhr" can be replaced with "Der Laden macht um zehn Uhr auf". That's because while its meaning looks like it were reflexive, it really is transitive: The actual meaning is: "Der Laden öffnet seine Türen um zehn Uhr", not "der Laden öffnet sich um zehn Uhr".
You could also say: Meagan öffnet die Wagentür und steigt ein.
I've read this thread several times and thought the answers sufficient. But now I'm thinking about why we prefer in normal spoken language "aufmachen" as in "Willst du dein Geschenk nicht aufmachen?". We might as well say "öffnen", but we don't. I'm a bit puzzled why, "öffnen" is shorter, nevertheless we prefer "aufmachen" which is longer by one syllable.
Probably it is the vowel ö, the articulation of which needs a lot of work compared to the vowel au and a. The ö-sound needs strong lip rounding, with a lot of muscle tension and raising the tongue in i-position. A lot of articulation work whereas "aufmachen" needs almost none.