I think one of the first things to consider is that simply writing data to a chip does not mean it’s going to work with the intended system. For example, you can definitely write your credit card number to the chip and it will be stored in binary format within the memory contents, but it’s a bit like writing your credit card number on a piece of paper and then trying to swipe that piece of paper through a credit card machine. Technically the data is there, but the entire methodology and standards and framework required to safely and securely communicate that data in an expected way to the terminal is not there.
To see more about payments check out Making payments with an implant - FAQs - Dangerous Things Forum
When it comes to NFC, now you’re talking about a standard with which several of the chip implants comply with. That means you can correctly write shareable NFC data to the memory contents of an NFC compatible chip and if written correctly, that data can be shared with things like NFC enabled smartphones and other types of devices.
Access control solutions are an entirely different matter. There are several different ways they work and being able to copy an existing access card or fob to an implant really depends on that source chip being of a certain type, and us having a compatible reprogrammable chip implant that can properly pretend to be or so-called emulate that kind of chip.
I don’t know if you’ve had a chance to read through the primer posts at Quick technology primer - Primers - Dangerous Things Forum but it’s a good place to start. I will try to expand those posts to include more in-depth information but there should be some basic content and videos linked within that I hope do a decent job of explaining the basics.