A small, purpose built chip. The GSM Association refers to it variously depending on audience. For example on their landing page for eSIMs they refer to it as a

A small, purpose built chip. The GSM Association refers to it variously depending on audience. For example on their landing page for eSIMs they refer to it as a Secure Element: https://www.gsma.com/esim/ https://www.gsma.com/esim/ You can get from that page to their technical standards which go into much more detail about hardware for eSIMs. STMicro makes Arm based ones: https://www.st.com/en/secure-mcus/st4sim-201m.html https://www.st.com/en/secure-mcus/st4sim-201m.html