Both solutions exist.
Physical SIM
This is the classic solution: a small SIM card is installed in the phone.
eSIM
If your smartphone is compatible, eSIM is often more convenient.
No physical card is needed: the line is installed digitally on the phone.
MobiFone, for example, offers eSIM with digital activation and management, including through its app.
Viettel has also improved its system: since August 2026, the operator offers automated eSIM activation on compatible iOS devices that no longer requires scanning a QR code.