To a great extent, what the Garageband fingerpicking simulations are what's known as "rolls".
To perform a roll, you hold down a chord shape, and pick the strings in a particular order. For example, hold down an open G chord, and pick:
- G on the low E string for a crotchet
- G on the high E string for a crotchet
- B on the A string for a quaver
- B on the B string for a quaver
- D on the D string for a quaver
- G on the G string for a quaver
... and repeat.
When you change chord, you change shape with your left hand. You might continue playing the same right-hand roll, or you might switch to a different pattern. Typically you'd be trying to get root notes at key beats in each bar.
More advanced rolls involve some extra movement in the left hand, and GarageBand augments the rolls with occasional fills. There's no simple answer to this, except to play the notes in the right order with the right timing!
There are books full of rolls, and lots of videos on YouTube.