SIM card transferrable between iPhone models? My iPhone SE is not working anymore and someone has offered me an older (2015 - she doesn't know the model) iPhone that her husband bought shortly before his death.

Can I transfer my SIM card (Deutsche Telecom)from my iPhone SE into any other model of iPhone?  I'm assuming it's unlocked as he used it before his death.
Yes. If you are not using a carrier locked iPhone and using a standard SIM card, it can be easily transferred from one iPhone to another.

However, newer iPhones support cellular and connectivity technologies which may not be available on older models. For e.g. iPhone 5s and earlier lack support for 4G networking. You may wish to keep this in mind.