R controlled vowels are often called “Bossy R” because the r takes over and makes the vowel make a new sound.
The er, ir, and ur all make the same sound /er/ as in her, bird, and fur.
But ar and or are a little different, as they have more than one sound.
Ar usually makes the sound in car, but can also make the sound in beggar and arrow.
Or usually makes the sound in corn, but can also make the /er/ sound as in the word doctor.