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.