On the topic of NFC: my iPhone ApplePay thing taps so much more reliably than any of my credit or debit cards. Is this because it has its own power supply and doesn’t have to first be powered up by the machine?
The primary reason is probably just that the secure element in more recent iPhones is probably just an order of magnitude more beefy than the IC in your physical cards.
Powering up the IC actually doesn't take long, but the processing itself can: Contactless payment transactions (mostly) use asymmetric cryptography, and old one at that too (usually RSA), so simply crunching the numbers takes these fairly underpowered ICs quite some time, even when they include cryptographic coprocessors.
Compare that with (symmetric key based) transit ticket authentication, e.g. for MIFARE DESfire or Japanese Felica cards: These usually use DES or AES, which is lightning fast in comparison.
NFC supports passive mode (where one side is powered and the card is not) and active mode (where both sides are powered). So, yes, your phone is probably more reliable because it provides a powered data transmission.
An NFC card doesn't actively transmit data. Instead, it sends data using "load modulation", where it switches a load across the antenna to change how much power it absorbs. The transmitter can detect this change in power, but the signal is extremely weak (80 decibels below the transmitted signal), so it's amazing that it works at all.
The iPhone doesn't actually use NFC's peer-to-peer/"active" mode (since contactless payments aren't an NFC application; see my other comments on that), but it does specifically include an NFC "field amplifier" IC (shown in some iFixit teardowns), which most other smartphones and of course all physical cards/tags lack.
This does mean that iPhones can't do cool tricks like booting up the secure element purely from the field with a completely dead battery though that some earlier Android and Windows Phones could do (or at least Apple has intentionally deactivated that capability for a more consistent/secure experience) :)
Additional conjecture: a device with upgradable software can take advantage of updates to readers and protocols. Whereas the physical card is stuck at the version it was created with.
