Thank you AugieTN and Fox Mulder for your replies.
1. Power (W) = Voltage (V) * Current (A). Knowing the maximum input power (watts) supported, one cannot know the maximum input voltage supported. The example given by Fox Mulder is where the charger has options of 9V or 5V output [Image attached] , so maybe you can use the term "backwards compatible" for such chargers where the charger will be automatically set to 5V output for phones which support only 5V input charging. But, as an example, if the charger has ONLY 9V or 12V or 20V output and the android phone has ONLY 5V input charging supported then this will damage the battery.
2. [Ways to know maximum input power/voltage/current supported] I have two phones currently. First, Realme Narzo 30 Pro 5G:
Realme Narzo 30 Pro 5G - Full phone specifications where 30W is stated. Some similar sites have also stated that maximum input power is 30W. I have the original charger which has output: "5V * 2A or 5V * 6A Max" so fastest charging (5V * 6A = 30W) is possible and
hence I also know that at least 5V input voltage is supported. But whether higher voltages are supported, that I don't know. Second phone I have is Samsung Galaxy On8 (quite an old phone):
Samsung Galaxy On8 - Full phone specifications where maximum input power/voltage/current supported is NOT stated. I have also checked specs for Samsung Galaxy On8 in other sites but maximum input power/voltage/current supported is again NOT stated. Also I don't have the original charger. In both the phones I can't find the maximum power/voltage/current supported in any Android Settings, Battery apps, Device Info apps etc. Hence for the second phone (Samsung Galaxy On8), I can't find a way to know the maximum input power/voltage/current supported!!
@Fox Mulder: Which Android Version are you using on your Samsung phone?
Thank you.