Kotlin codes are compiled to JVM byte-code(this is a java kind of code that can be interpreted by JVM machine). The JVM machine is what interpret this JVM byte-code. This JVM machine is found on all android device.
When you install the app on your phone, the codes in the app is interpreted and compilled by AOT compiller (Ahead of time compiller) and saves it on your phone. This is a one time operation.
But when you perform regular operations on your phone like chatting and uploading pictures, the codes are compilled by JIT compiler(Just in Time). This operations happens regularly.
NOTE: The Above Explanation is Targeted for Beginners.