

Radio is referring to the proprietary piece of hardware that is in every cell phone that handles the physical portion of transmitting radio waves for wireless connectivity. This piece of hardware generally handles LTE/5G, wifi, and bluetooth.
Your IMEI is the identifier that is burned into that hardware, which uniquely lets the carriers determine if your device can authenticate on their networks.
You can turn off the radio by disabling your cellular network or turning on airplane mode on your phone. You will temporarily lose access to your cellular network until you turn it back on.
Newer devices don’t require SIM cards to connect. They have eSIM cards that can be used. You still need one or the other to connect to a cellular network.
Yes, each eSIM has its own unique IMEI. Some phones have multiple eSIMs, so they could technically have multiple carriers/phone numbers assigned to it.