> Does USB-C not have a way to indicate that a potential power source is a battery so that the MacBook can charge it if it's plugged in to power, and reverse roles otherwise? Is this a fault how the power bank or macOS implements power negotiation, or is this scenario simply unaddressed in USB-C?
USB-C does support this. It's known as a DRP (Dual Role Port). USB PD can be used to signal switching between downstream and upstream facing ports depending on state of charge of the battery for instance. The problem is that many devices do not support this, and I strongly suspect your power bank is the issue here. iPhones and Apple stuff in general supports DRP renegotiation quite well. They tend to be USB-C compliant as much as possible, which can lead to issues with interfacing with 'USB-C' devices that are not actually properly compliant.
I'd have thought that iPhones would have sane defaults with USB-C, but it's a real pain when using USB-C to provide tethered internet to a laptop. No, I don't want the phone to charge the laptop, they both have batteries of their own
And there's seemingly no way to get the phone to not try to charge things regardless of what the other side thinks (and when plugging the laptop in, it starts to charge the phone).
The alternative would be using WiFi (in a very RF polluted space) or Bluetooth (horribly slow), versus USB-C where 5g via my phone in Bangkok can get 250mbit easily. Whereas my Android phone has options for 'data only' etc. without charging which seems like it's more of a UX 'Apple' thing than anything else.
USB-C does support this. It's known as a DRP (Dual Role Port). USB PD can be used to signal switching between downstream and upstream facing ports depending on state of charge of the battery for instance. The problem is that many devices do not support this, and I strongly suspect your power bank is the issue here. iPhones and Apple stuff in general supports DRP renegotiation quite well. They tend to be USB-C compliant as much as possible, which can lead to issues with interfacing with 'USB-C' devices that are not actually properly compliant.