Garageband—how to change volume of a region (not whole track)?
Finally found the answer, not very intuitive but cool once you know.

There's a button to toggle 'automation'. Once that's on you can select from a dropdown whether you want to change volume, echo, reverb, etc. Then you click on the actual waves on the track and a coloured line will appear. Clicking along the line creates break points, and you can move either points or the line segments between points to shift the volume. It's not based on regions—you can adjust it anywhere.

For more information:
https://www.howtogeek.com/238649/how-to-use-automation-in-garageband/