To add your Dave debit card to your virtual wallet, navigate to Dave Checking, then tap View debit card, then tap Add to Apple Wallet/G Pay.
You can go to your phone's settings to make Dave your default card.
For iPhone, go to your phone's Settings, then Wallet & Apple Pay, then under Transaction Defaults set Dave as default.
For Android, go to your phone's G Pay app, then tap Payment, then tap on your Dave card to make it the default.