A SIM Card* is a ISO card running a variant of JavaCard OS with standard APIs mandated by GSMA or 3GPP (despite it's already 5G) implemented, but carriers can (

A SIM Card* is a ISO card running a variant of JavaCard OS with standard APIs mandated by GSMA or 3GPP (despite it's already 5G) implemented, but carriers can (and do) add internal features for their use, to the point that some weird SIMs (no longer seen today) could also function as a debit card¤. Think of it as roughly similar to the POSIX compatibilty between Linux, macOS and *BSD, while having widly different capabilities tacked on. eSIMs just remove the physical ISO card package and put it instead into a dedicated chip inside your phone, so in theory and in practice it could do anything a regular SIM can do. * To be fair, I'm not familiar with how CDMA networks handle this, but this is irrelevant in the sense that CDMA (depending on the carrier) is dying or even dead already (in favour of LTE et al.). ¤ Payment cards and original SIM cards are ISO/IEC Card ID-1, the "mini" SIM cards are ISO/IEC Card ID-000, and micro and nano SIM cards are non-ISO additions by 3GPP.