Charging specs are fairly complicated so it is hard to say. Even if your Android phone for example supports faster charging than the iPhone (not hard, the iPhone is one of the slowest charging flagship devices around), it may not support that specific Power Delivery standard. My iPhone for example supports 15W wireless charging. Even though I had a few chargers that supported 15W charging, the iPhone uses a specific PD profile for that and I needed to buy a new charger that supported it to make sure I got the full speed.

So basically without knowing what charger, what specific Android and what specific iPhone you're using, it is impossible to say. I find it unlikely that the charger itself knows it is connected to an iPhone, let alone would it prioritize it.

Just a note, I'd recommend against fast charging unless you need it. It's not great for the battery of the phone. I know my phone automatically slow charges when I plug it up at night because it assumes it will stay plugged up for a few hours.

Are you always plugging hers in second? Maybe it’s just giving priority to the most recent thing plugged in.

AFAIK, Xiaomi chargers offer more current over voltage (which is locked to 5V). Other Android phones may charge even faster than Xiaomis and iPhones, provided the charger implements modern standards with variable voltage.

There are a number of factors involved. Each phone will support a number of protocols and charging speeds, as will the charger. The two devices will typically perform a handshake to negotiate a charging speed before charging begins. This is what allows the voltage to be adjusted, for example to 9/12/24V rather than the 5V default. Then you have to consider the maximum power to be delivered. Some devices might negotiate a higher power, which at the same voltage will result in slower charging.

Tldr on voltage vs power vs current: power = voltage * current. You want to minimize current in the long wires and connectors/ contacts where there is high resistance because resistance causes power loss, so you use higher voltage until it reaches the end device before dropping it back down.

If the Android device negotiated 18W and the apple device 25W for example (idk what apple does) then the apple device will charge faster. Also, if the apple device has a smaller battery, it will charge faster. You may be able to get faster charging with a different combination of port/cable, charger, or settings on the Android device. For example, I restrict my phone to charge to at most 85% and slow charge because it keeps the battery from degrading as quickly, so I can still use my phone in several year's time without having to recharge it every few hours.

Are both phones USB-C? If so there’s probably a difference in the advertised capabilities.

Another option is that the BMS in your phone is just more careful than Apple’s.

Your Android may be crap