You’ll need to create a playback template, and then set that playback template to be the default for new projects on the Play page of Preferences. That sounds simple, but there’s quite a bit to it.
The first question: which sounds do you want to use? The Garittan Instruments For Finale? GPO5? Something else?
You’ll then need to create a project that contains the instruments whose sounds you want to change, manually assign the ARIA Player and load the appropriate sounds for each one, then specify an appropriate expression map if one is needed (at the least you might need to choose between using velocity or a MIDI controller for dynamics).
Once you’ve done that for each instrument, you save an endpoint configuration that saves the mapping between the instruments in your score, the ARIA Player configuration, and the expression maps to use.
One or more endpoint configurations can then be combined into a playback template, so that Dorico knows how to load up the presets you’ve saved when you add instruments, and knows how to drive your ARIA sounds.
This is reasonably advanced stuff, and my advice would be to take a bit of time to get used to the overall Dorico experience before you invest a lot of time in creating playback templates. It’s not generally something I would recommend for somebody in their first week or two of using the software!