Sim stands for Subscriber Identification Module. It is a small chip that contains information about the user and enables a device to connect to the cellular network. Thing to consider is that it is a physical chip and separate from the cellphone. It has 3 different sizes that were released over different times. Mini, Micro and Nano
eSIM are Embedded Subscriber Identification Module. They are also physical chips but are embedded on to the board of the cellphone. These sims cannot be removed from the phones but can be modified through phone settings.
iSIM is Integrated Subscriber Identification Module. iSIM is a not a physical sim but the functionalities of the SIM built into the architecture of the phone’s system-on-chip (SoC) or processor. iSIM works the same way as the eSIM without the separate hardware and enjoys all the benefits of eSIM.