Question

for any substance why the temperature remains constant ​

Answer 1

Because at that temperature the amount of extra heat which is being supplied to the substance is used in Breaking the intermolecular forces so the temperature remains constant and the until the state of matter changes the heat is used to break intermolecular forces and hence after the breaking of intermolecular forces

Answer 2

The temperature of any substance remains constant unless any external heat is applied to it or its surrounding temperature changes to influence its temperature( substance temperature)