sim applications

If you mean the carrier provided Sim Applications, it can be found in different places depending on the iOS version. for iOS 12 its in Settings->Cellular->Sim Applications (last option in the section titled after your carrier):

For earlier iOS versions: Settings->Phone->Sim Applications

The option is entirely carrier based, and some carriers may not enable it.

If it's not visible under Settings -> Cellular -> Sim Applications your phone may be in Dual Mode: you can switch SIMs simply by dialing '001' (for SIM 1) or '002' (for SIM 2) on your phone. This option causes the STK menu to temporarily disappear - to retrieve the STK menu and exit '007 mode' dial '007' on your phone.

There is no support for SIM applications on an iPhone in any version. iPhones only use the SIM for the phone number.