How do I customize availability for specific appointment types that differ from my general availability?

The availability of each appointment template can be set using the Time Grid option
(Settings → Appointment Templates → select the desired template → "Time Grid" tab).

To do this, select the "Enter times individually" option.
Then edit the current week as desired by entering all desired start times individually – e.g., Monday: 8:00, 8:30, 9:00, 9:30, etc. This defines the time slots that should be offered to your customers for booking.

Once you have completed the weekly settings, you can carry them over to the remaining weeks until the end of the year or even until the end of the following year.