Short answer
HubSpot's KB: the mobile app imports contacts from device contacts, the business card scanner or a QR code. Card scanning works on Android and iOS 16 or higher, not on iPads. Scanned contacts get a business-card-scanner source drill-down.
1. Desktop imports
Bigger lists go through desktop import. My demo portal:

2. Events
Tag scanned contacts with a property so you can find them later.