Sim Card

Only your cellular network knows whether their existing SIMs will work for their 5G service. Call your network's support or Customer Service. Meanwhile, your existing SIM will work in your new iPhone, even if only on 4G.

Physically it possibly can, My 4 year old O2 SIM that started life in my Samsung then passed through iphones 6S, 7 and 12 can receive 5G but only because O2 uprated my account.