Expert level: Mix master
On iOS, use the ‘Mixer’ menu to tweak echo, reverb, treble, bass and compressor effects for a selected track.
On macOS, you get more options — open Smart Controls (B), and expand the Plug-ins menu. You can load and adjust up to four plug-ins for the instrument.
Load Audio Units
You can expand GarageBand with AU (Audio Unit) plug-ins. If a compatible one is installed on your Mac, click the Instrument slot in Plug-ins, and select it from the menu. Its settings can be adjusted in its pop-up window.
On iOS, create an External track and select Audio Unit Extensions. Choose an AU to load it. As of iOS 11, AUs can take over the entire play area, so you can access the likes of Model 15’s custom keyboards.
Try Inter-App Audio
For those iOS audio apps you love that don’t yet have Audio Unit capabilities, there’s another option. When creating an External track, choose Inter-App Audio. This enables you to record third-party synths/audio apps directly into a GarageBand track, albeit with an end result that’s a rendered audio waveform, not editable MIDI data.
And for those apps that don’t support Inter-App Audio either, there’s the more widely supported (along with flexible and usable) Audiobus (RM39.90).
Use the app to define input, effects and output chains (GarageBand as output if recording to it), fire up your other music app, and use Audiobus’s overlay menu to trigger recordings. Again, these are rendered as audio waveforms.