Your default destination is the one new aliases forward to automatically when you don't pick one during alias creation. It's also used as the fallback if an alias's assigned destination is ever removed.
Changing the default
Go to Destinations. Find the destination you want to make the default and open its menu (three dots). Choose Set as default.
The star icon on the destination list shows which one is currently your default.
What changes when you switch the default
Only future aliases are affected — any aliases you've already created keep forwarding to whichever destination they were assigned to. Changing the default doesn't retroactively reassign existing aliases.
If you want existing aliases to use a different destination, you'll need to edit each one individually from Aliases.
You always need a default
You can't remove the default status from a destination without first setting another one as default. If you only have one destination, it's the default by definition and can't be unset.
Tip
If you primarily use one inbox for everything, set that as your default and you'll rarely need to touch the destination selector when creating new aliases.